Clarify error for slow `cpu_sup` port init
Clarify error for slow `cpu_sup` port init

I noticed that running an R16B03-1 node on an overloaded host produced
log entries like the following ones:

```
2014-08-22 21:52:31 =ERROR REPORT====
Error in process <0.24112.3> on node '...@127.0.0.1' with exit value: {{case_clause,{data,4711}},[{cpu_sup,get_uint32_measurement,2,[{file,"cpu_sup.erl"},{line,227}]},{cpu_sup,measurement_server_loop,1,[{file,"cpu_sup.erl"},{line,585}]}]}
```

```
===== ALIVE Fri Aug 22 21:50:14 CEST 2014
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed

===== ALIVE Fri Aug 22 22:07:46 CEST 2014
```

I performed a code inspection on the `cpu_sup` module and I concluded
that the `case_clause` error shows a small issue in the `cpu_sup`
module, that happens when the port used in `cpu_sup` is slow to start
- as it may happen on an overloaded node.

The `cpu_sup` `gen_server` process keeps in its state the pid of an
unlinked process (called "measurement server" - see `cpu_sup:init/1`),
in order to do dirty stuff (e.g. reading the filesystem, running OS
commands) and start_link-ing & managing the connected process to a
port (called "port server" - see `measurement_server_init/0`). So the
process organization looks like this:

```
cpu_sup - measurement server - port server - port
```

When the measurement server start_links the port server (see
`port_server_start/0`) it sends a `{self(), ?ping}` message to it and
expects an answer within 6s, otherwise it returns `{error, timeout}`
rather than the pid. This has two issues:

* The measurement server keeps `{error, ...}` in its state as if it
  were a pid - that makes no sense;

* A late `{Pid, {data,4711}}` response may arrive in the mailbox of
  the measurement server, that will believe it to be a request to be
  processed, causing a `case_clause` error.

This commit teaches the measurement server to check the success of the
initialization of the port server by matching on the return value of
`port_server_start/0` (renamed to `port_server_start_link/0` for the
sake of clarity) in order to fail earlier and with an error clearer
than `{case_clause,{data,4711}}`. In such case I expect the
measurement server to be restarted by the `cpu_sup` `gen_server` (see
`handle_call/3`) - as before.

BTW It is not clear to me when the `handle_info({'EXIT', _Port,
Reason}, State)` may be called (the `cpu_sup` `gen_server` does not
link to the measurement server) but I am leaving it.

msantos/otp,RGafiyatullin/otp,gjaldon/otp,schlagert/otp,stolen/otp,isvilen/otp,massemanet/otp,sitexa/otp,vinoski/otp,getong/otp,aboroska/otp,lemenkov/otp,uabboli/otp,bsmr-erlang/otp,fenollp/otp,neeraj9/otp,mikpe/otp,release-project/otp,getong/otp,paladim/otp,riverrun/otp,falkevik/otp,cnbin/otp,msantos/otp,bsmr-erlang/otp,schlagert/otp,rlipscombe/otp,isvilen/otp,hairyhum/otp,benoitc/otp-1,uabboli/otp,sammoth-wazoku/otp,gjaldon/otp,cobusc/otp,lightcyphers/otp,bsmr-erlang/otp,NOMORECOFFEE/otp,msantos/otp,matwey/otp,lantti/otp,bsmr-erlang/otp,yangchengjian/otp,goertzenator/otp,enikki/otp,ahmedshafeeq/otp,entropiae/otp,vladdu/otp,tuncer/otp,goertzenator/otp,mikpe/otp,getong/otp,lhslll/otp,RaimoNiskanen/otp,bjorng/otp,ferd/otp,bugs-erlang-org/otp,schlagert/otp,jj1bdx/otp,platinumthinker/otp,lianghaivv/otp,falkevik/otp,g-andrade/otp,sitexa/otp,mujiatong/otp,cnbin/otp,yangchengjian/otp,c-rack/otp,release-project/otp,Teino1978-Corp/otp,msantos/otp,jinshana/otp,haguenau/otp,neeraj9/otp,weisslj/otp,benoitc/otp-1,vladdu/otp,ahmedshafeeq/otp,dumbbell/otp,beni55/otp,mujiatong/otp,yangchengjian/otp,dgud/otp,dgud/otp,palas/otp,NOMORECOFFEE/otp,RJ/otp,emile/otp,lantti/otp,electricimp/otp,g-andrade/otp,mujiatong/otp,cobusc/otp,ader1990/otp,krishnakumar4a4/otp,paulcager/otp,lucafavatella/otp,jj1bdx/otp,ferd/otp,theom/otp,paulcager/otp,emacsmirror/erlang,haguenau/otp,emacsmirror/erlang,jj1bdx/otp,theom/otp,getong/otp,klarna/otp,bsmr-erlang/otp,neeraj9/otp,krishnakumar4a4/otp,mikpe/otp,potatosalad/otp,legoscia/otp,lhslll/otp,matwey/otp,ferd/otp,awetzel/otp,lantti/otp,paladim/otp,vinoski/otp,weisslj/otp,cobusc/otp,lhslll/otp,kvakvs/otp,goertzenator/otp,sdebnath/otp,platinumthinker/otp,entropiae/otp,g-andrade/otp,basho/otp,emile/otp,GinjaNinja32/otp,saleyn/otp,paladim/otp,getong/otp,lightcyphers/otp,psyeugenic/otp,RichMorin/otp,enikki/otp,vic/otp,palas/otp,c-rack/otp,getong/otp,marquisthunder/otp,jamesruan/otp,basho/otp,electricimp/otp,tuncer/otp,paulcager/otp,emacsmirror/erlang,weisslj/otp,RGafiyatullin/otp,RoadRunnr/otp,neeraj9/otp,bsmr-erlang/otp,emacsmirror/erlang,rlipscombe/otp,sammoth-wazoku/otp,lrascao/otp,bugs-erlang-org/otp,erlang/otp,haguenau/otp,falkevik/otp,RichMorin/otp,mikpe/otp,marquisthunder/otp,awetzel/otp,vladdu/otp,stolen/otp,emacsmirror/erlang,uabboli/otp,potatosalad/otp,electricimp/otp,vinoski/otp,hairyhum/otp,platinumthinker/otp,ader1990/otp,lucafavatella/otp,Teino1978-Corp/erlang-otp,beni55/otp,weisslj/otp,RaimoNiskanen/otp,theom/otp,electricimp/otp,RaimoNiskanen/otp,Teino1978-Corp/otp,jamesruan/otp,jemsbhai/otp,RaimoNiskanen/otp,weisslj/otp,platinumthinker/otp,massemanet/otp,jinshana/otp,yangchengjian/otp,release-project/otp,lightcyphers/otp,cnbin/otp,jinshana/otp,fenollp/otp,ferd/otp,VincentHHL/otp,bugs-erlang-org/otp,vinoski/otp,gjaldon/otp,hairyhum/otp,schlagert/otp,dgud/otp,enikki/otp,electricimp/otp,saleyn/otp,matwey/otp,jamesruan/otp,bsmr-erlang/otp,sammoth-wazoku/otp,emile/otp,bernardd/otp,lantti/otp,bsmr-erlang/otp,electricimp/otp,uabboli/otp,psyeugenic/otp,derek121/otp,RaimoNiskanen/otp,ahmedshafeeq/otp,lrascao/otp,tuncer/otp,sdebnath/otp,mikpe/otp,lemenkov/otp,paulcager/otp,potatosalad/otp,palas/otp,fenollp/otp,bugs-erlang-org/otp,mujiatong/otp,tuncer/otp,RGafiyatullin/otp,c-rack/otp,dgud/otp,stolen/otp,RJ/otp,lightcyphers/otp,entropiae/otp,lucafavatella/otp,bjorng/otp,RaimoNiskanen/otp,lantti/otp,benoitc/otp-1,dumbbell/otp,goertzenator/otp,derek121/otp,marquisthunder/otp,tuncer/otp,krishnakumar4a4/otp,marquisthunder/otp,isvilen/otp,lucafavatella/otp,fenollp/otp,stolen/otp,emile/otp,haguenau/otp,legoscia/otp,erlang/otp,RichMorin/otp,bjorng/otp,NOMORECOFFEE/otp,RGafiyatullin/otp,dumbbell/otp,jamesruan/otp,massemanet/otp,release-project/otp,potatosalad/otp,getong/otp,GinjaNinja32/otp,Teino1978-Corp/otp,bernardd/otp,hairyhum/otp,enikki/otp,lhslll/otp,bugs-erlang-org/otp,dumbbell/otp,cnbin/otp,vinoski/otp,isvilen/otp,RJ/otp,dumbbell/otp,mikpe/otp,beni55/otp,rlipscombe/otp,kvakvs/otp,ferd/otp,mujiatong/otp,vinoski/otp,jj1bdx/otp,saleyn/otp,massemanet/otp,RGafiyatullin/otp,stolen/otp,palas/otp,lianghaivv/otp,lucafavatella/otp,dgud/otp,awetzel/otp,vic/otp,emile/otp,paulcager/otp,beni55/otp,uabboli/otp,NOMORECOFFEE/otp,paulcager/otp,theom/otp,c-rack/otp,entropiae/otp,enikki/otp,lightcyphers/otp,platinumthinker/otp,cobusc/otp,jj1bdx/otp,RaimoNiskanen/otp,RJ/otp,dumbbell/otp,gjaldon/otp,jinshana/otp,bjorng/otp,NOMORECOFFEE/otp,RichMorin/otp,GinjaNinja32/otp,schlagert/otp,bernardd/otp,erlang/otp,bjorng/otp,hairyhum/otp,massemanet/otp,riverrun/otp,sdebnath/otp,hairyhum/otp,matwey/otp,psyeugenic/otp,cnbin/otp,ahmedshafeeq/otp,fenollp/otp,aboroska/otp,jinshana/otp,sdebnath/otp,legoscia/otp,beni55/otp,VincentHHL/otp,klarna/otp,uabboli/otp,johanclaesson/otp,stolen/otp,mujiatong/otp,tuncer/otp,aboroska/otp,ahmedshafeeq/otp,basho/otp,lucafavatella/otp,neeraj9/otp,ahmedshafeeq/otp,marquisthunder/otp,RichMorin/otp,sammoth-wazoku/otp,fenollp/otp,cobusc/otp,platinumthinker/otp,RoadRunnr/otp,matwey/otp,matwey/otp,vladdu/otp,kvakvs/otp,haguenau/otp,goertzenator/otp,basho/otp,lemenkov/otp,klarna/otp,g-andrade/otp,NOMORECOFFEE/otp,electricimp/otp,fenollp/otp,cnbin/otp,saleyn/otp,jemsbhai/otp,lantti/otp,awetzel/otp,yangchengjian/otp,jemsbhai/otp,potatosalad/otp,GinjaNinja32/otp,benoitc/otp-1,rlipscombe/otp,kvakvs/otp,basho/otp,legoscia/otp,sammoth-wazoku/otp,krishnakumar4a4/otp,jemsbhai/otp,RGafiyatullin/otp,aboroska/otp,potatosalad/otp,entropiae/otp,sdebnath/otp,matwey/otp,johanclaesson/otp,vic/otp,bugs-erlang-org/otp,VincentHHL/otp,uabboli/otp,hairyhum/otp,derek121/otp,lemenkov/otp,release-project/otp,awetzel/otp,bugs-erlang-org/otp,klarna/otp,aboroska/otp,rlipscombe/otp,lantti/otp,getong/otp,paladim/otp,dgud/otp,palas/otp,marquisthunder/otp,msantos/otp,rlipscombe/otp,falkevik/otp,jamesruan/otp,VincentHHL/otp,VincentHHL/otp,krishnakumar4a4/otp,schlagert/otp,falkevik/otp,bernardd/otp,krishnakumar4a4/otp,rlipscombe/otp,release-project/otp,rlipscombe/otp,RichMorin/otp,ferd/otp,Teino1978-Corp/erlang-otp,ader1990/otp,jinshana/otp,benoitc/otp-1,RoadRunnr/otp,krishnakumar4a4/otp,kvakvs/otp,electricimp/otp,johanclaesson/otp,haguenau/otp,Teino1978-Corp/otp,riverrun/otp,johanclaesson/otp,paladim/otp,goertzenator/otp,emile/otp,vladdu/otp,GinjaNinja32/otp,uabboli/otp,vic/otp,jemsbhai/otp,ader1990/otp,neeraj9/otp,vic/otp,haguenau/otp,ahmedshafeeq/otp,awetzel/otp,bernardd/otp,isvilen/otp,tuncer/otp,lianghaivv/otp,lemenkov/otp,cobusc/otp,psyeugenic/otp,emacsmirror/erlang,neeraj9/otp,johanclaesson/otp,jamesruan/otp,dumbbell/otp,potatosalad/otp,benoitc/otp-1,g-andrade/otp,johanclaesson/otp,ferd/otp,VincentHHL/otp,yangchengjian/otp,theom/otp,dgud/otp,jj1bdx/otp,emile/otp,lightcyphers/otp,RoadRunnr/otp,emile/otp,psyeugenic/otp,jinshana/otp,massemanet/otp,awetzel/otp,erlang/otp,fenollp/otp,lrascao/otp,msantos/otp,emile/otp,saleyn/otp,psyeugenic/otp,aboroska/otp,tuncer/otp,falkevik/otp,legoscia/otp,bernardd/otp,riverrun/otp,klarna/otp,Teino1978-Corp/erlang-otp,lantti/otp,palas/otp,weisslj/otp,c-rack/otp,jemsbhai/otp,mikpe/otp,NOMORECOFFEE/otp,ader1990/otp,riverrun/otp,vladdu/otp,dgud/otp,release-project/otp,psyeugenic/otp,mikpe/otp,RoadRunnr/otp,lianghaivv/otp,vic/otp,derek121/otp,bjorng/otp,bjorng/otp,platinumthinker/otp,jemsbhai/otp,NOMORECOFFEE/otp,basho/otp,schlagert/otp,matwey/otp,vic/otp,palas/otp,getong/otp,lhslll/otp,gjaldon/otp,neeraj9/otp,legoscia/otp,falkevik/otp,johanclaesson/otp,bernardd/otp,dumbbell/otp,sdebnath/otp,matwey/otp,riverrun/otp,yangchengjian/otp,derek121/otp,benoitc/otp-1,sammoth-wazoku/otp,lianghaivv/otp,isvilen/otp,erlang/otp,bjorng/otp,theom/otp,potatosalad/otp,bjorng/otp,bugs-erlang-org/otp,release-project/otp,aboroska/otp,legoscia/otp,lhslll/otp,lianghaivv/otp,massemanet/otp,jemsbhai/otp,lhslll/otp,stolen/otp,lightcyphers/otp,lhslll/otp,gjaldon/otp,paladim/otp,enikki/otp,enikki/otp,basho/otp,getong/otp,riverrun/otp,Teino1978-Corp/erlang-otp,falkevik/otp,msantos/otp,yangchengjian/otp,GinjaNinja32/otp,vladdu/otp,ahmedshafeeq/otp,vinoski/otp,RoadRunnr/otp,lucafavatella/otp,cnbin/otp,Teino1978-Corp/otp,aboroska/otp,benoitc/otp-1,platinumthinker/otp,psyeugenic/otp,isvilen/otp,lrascao/otp,ferd/otp,riverrun/otp,sitexa/otp,mikpe/otp,jamesruan/otp,msantos/otp,gjaldon/otp,sitexa/otp,paulcager/otp,emacsmirror/erlang,legoscia/otp,vinoski/otp,mujiatong/otp,jj1bdx/otp,lianghaivv/otp,beni55/otp,theom/otp,saleyn/otp,RaimoNiskanen/otp,hairyhum/otp,erlang/otp,paladim/otp,weisslj/otp,cnbin/otp,bernardd/otp,paulcager/otp,tuncer/otp,lemenkov/otp,dgud/otp,mikpe/otp,uabboli/otp,theom/otp,sitexa/otp,weisslj/otp,erlang/otp,kvakvs/otp,beni55/otp,schlagert/otp,erlang/otp,lrascao/otp,RJ/otp,saleyn/otp,cobusc/otp,saleyn/otp,entropiae/otp,lucafavatella/otp,lemenkov/otp,cobusc/otp,bsmr-erlang/otp,RGafiyatullin/otp,gjaldon/otp,kvakvs/otp,marquisthunder/otp,RoadRunnr/otp,vinoski/otp,klarna/otp,c-rack/otp,VincentHHL/otp,kvakvs/otp,mujiatong/otp,g-andrade/otp,ader1990/otp,stolen/otp,lemenkov/otp,marquisthunder/otp,sitexa/otp,aboroska/otp,palas/otp,RichMorin/otp,Teino1978-Corp/erlang-otp,dumbbell/otp,derek121/otp,bernardd/otp,jamesruan/otp,Teino1978-Corp/erlang-otp,jj1bdx/otp,derek121/otp,ader1990/otp,RGafiyatullin/otp,VincentHHL/otp,lightcyphers/otp,jj1bdx/otp,ahmedshafeeq/otp,c-rack/otp,RJ/otp,vinoski/otp,g-andrade/otp,dumbbell/otp,kvakvs/otp,sammoth-wazoku/otp,GinjaNinja32/otp,dgud/otp,weisslj/otp,g-andrade/otp,basho/otp,ader1990/otp,enikki/otp,c-rack/otp,lrascao/otp,entropiae/otp,beni55/otp,fenollp/otp,potatosalad/otp,entropiae/otp,lrascao/otp,goertzenator/otp,vladdu/otp,derek121/otp,jj1bdx/otp,g-andrade/otp,emacsmirror/erlang,Teino1978-Corp/erlang-otp,sammoth-wazoku/otp,haguenau/otp,sitexa/otp,isvilen/otp,Teino1978-Corp/otp,isvilen/otp,electricimp/otp,sdebnath/otp,vladdu/otp,lrascao/otp,goertzenator/otp,massemanet/otp,rlipscombe/otp,RichMorin/otp,GinjaNinja32/otp,potatosalad/otp,legoscia/otp,isvilen/otp,release-project/otp,RJ/otp,RoadRunnr/otp,jinshana/otp,emacsmirror/erlang,sitexa/otp,sdebnath/otp,johanclaesson/otp,goertzenator/otp,lrascao/otp,erlang/otp,falkevik/otp,vic/otp,paladim/otp,saleyn/otp,rlipscombe/otp,krishnakumar4a4/otp,g-andrade/otp,awetzel/otp,RJ/otp,klarna/otp,erlang/otp,bjorng/otp,massemanet/otp,ferd/otp,Teino1978-Corp/otp,RoadRunnr/otp,Teino1978-Corp/erlang-otp,klarna/otp,basho/otp,Teino1978-Corp/otp,RaimoNiskanen/otp,emacsmirror/erlang,lianghaivv/otp