don't crash if ~/Library/Logs/DiagnosticReports does not exist.
don't crash if ~/Library/Logs/DiagnosticReports does not exist.

This was pretty bad - xctool would actually crash if this directory
didn't exist (meaning you had no crash reports on your system).

Fixes issue #11

Tested with...

```
./xctool.sh -workspace
xctool/xctool-tests/TestData/TestWorkspace-Library/TestWorkspace-Library.xcworkspace
-scheme TestProject-Library -configuration Debug -arch i386 -sdk
iphonesimulator test -only TestProject-LibraryTests
```

n054/xctool,theiren46/xctool,adamjernst/xctool,kcoons/xctool,soundcloud/xctool,lqi/xctool,Richardlihui/xctool,githubdelegate/xctool,facebook/xctool,soundcloud/xctool,AutoScout24/xctool,kcoons/xctool,PSPDFKit-labs/xctool,AutoScout24/xctool,FiftyThree/xctool,0xced/xctool,theiren46/xctool,lpios/xctool,adamjernst/xctool,githubdelegate/xctool,trello/xctool,Richardlihui/xctool,soundcloud/xctool,FiftyThree/xctool,leopardpan/xctool,LegNeato/xctool,n054/xctool,trello/xctool,bgertzfield/xctool,facebook/xctool,duk42111/xctool,ACChe/xctool,lpios/xctool,ekingo/xctool,LegNeato/xctool,SuPair/xctool,PSPDFKit-labs/xctool,bhamiltoncx/xctool,PSPDFKit-labs/xctool,AutoScout24/xctool,kamoljan/xctool,LegNeato/xctool,theiren46/xctool,modulexcite/xctool,jspahrsummers/xctool,dvor/xctool,agreco/xctool,kernandreas/xctool,duk42111/xctool,FiftyThree/xctool,modulexcite/xctool,PSPDFKit-labs/xctool,dvor/xctool,modulexcite/xctool,lpios/xctool,n054/xctool,1yvT0s/xctool,adamjernst/xctool,bhamiltoncx/xctool,jspahrsummers/xctool,ekingo/xctool,bhamiltoncx/xctool,facebook/xctool,1yvT0s/xctool,lqi/xctool,ekingo/xctool,0xced/xctool,SuPair/xctool,Richardlihui/xctool,1yvT0s/xctool,SuPair/xctool,leopardpan/xctool,blighli/xctool,SuPair/xctool,yaoxiaoyong/xctool,blighli/xctool,blighli/xctool,leopardpan/xctool,adamjernst/xctool,bhamiltoncx/xctool,duk42111/xctool,n054/xctool,leopardpan/xctool,facebook/xctool,trello/xctool,theiren46/xctool,LegNeato/xctool,modulexcite/xctool,duk42111/xctool,dvor/xctool,soundcloud/xctool,Richardlihui/xctool,jspahrsummers/xctool,kcoons/xctool,dvor/xctool,AutoScout24/xctool,blighli/xctool,yaoxiaoyong/xctool,ACChe/xctool,bgertzfield/xctool,FiftyThree/xctool,lqi/xctool,ACChe/xctool,soundcloud/xctool,agreco/xctool,facebook/xctool,yaoxiaoyong/xctool,ACChe/xctool,kamoljan/xctool,Monits/xctool,bhamiltoncx/xctool,bgertzfield/xctool,n054/xctool,Monits/xctool,yaoxiaoyong/xctool,kamoljan/xctool,lpios/xctool,1yvT0s/xctool,kamoljan/xctool,githubdelegate/xctool,soundcloud/xctool,lpios/xctool,ACChe/xctool,bgertzfield/xctool,ACChe/xctool,kernandreas/xctool,lpios/xctool,Monits/xctool,kernandreas/xctool,trello/xctool,kernandreas/xctool,n054/xctool,jspahrsummers/xctool,Monits/xctool,bgertzfield/xctool,githubdelegate/xctool,PSPDFKit-labs/xctool,facebook/xctool,ekingo/xctool,agreco/xctool,bhamiltoncx/xctool,bgertzfield/xctool,agreco/xctool