Improve the messages printed when buck self-updates
Improve the messages printed when buck self-updates

Summary:
(1.) When the .buckversion file specifies a revision that is not present
in the current repository, print a message stating that Buck is fetching
updates.

(2.) When the git fetch fails, only display a "failed" message.  Move the
bit about how the user can disable the update into the informational
message that was already displayed as described in (1.).

(3.) If the revision specified in .buckversion does not exist on the
remote, either because it's plain wrong or because the revision is on a
remote that is not configured in the local repository, the git checkout
fails. Catch the exception and display a message rather than a Python
Traceback.

Closes https://github.com/facebook/buck/pull/224

Test Plan:
Set .buckversion to a revision that does not exist. Run `buck clean` and
observe that it prints the message in (1.), then runs git fetch, and then
fails to check out the revision and fails with an error message.

rmaz/buck,stuhood/buck,mnuessler/buck,Heart2009/buck,tgummerer/buck,Addepar/buck,pwz3n0/buck,janicduplessis/buck,zhan-xiong/buck,Learn-Android-app/buck,mikekap/buck,Distrotech/buck,zpao/buck,nguyentruongtho/buck,sdwilsh/buck,1yvT0s/buck,Learn-Android-app/buck,dpursehouse/buck,romanoid/buck,rmaz/buck,vine/buck,ilya-klyuchnikov/buck,vine/buck,zhan-xiong/buck,MarkRunWu/buck,zhuxiaohao/buck,LegNeato/buck,artiya4u/buck,grumpyjames/buck,OkBuilds/buck,tgummerer/buck,robbertvanginkel/buck,robbertvanginkel/buck,shs96c/buck,siddhartharay007/buck,liuyang-li/buck,zhan-xiong/buck,Addepar/buck,vschs007/buck,mikekap/buck,neonichu/buck,LegNeato/buck,stuhood/buck,stuhood/buck,clonetwin26/buck,liuyang-li/buck,Distrotech/buck,janicduplessis/buck,darkforestzero/buck,rhencke/buck,daedric/buck,luiseduardohdbackup/buck,justinmuller/buck,sdwilsh/buck,tgummerer/buck,dushmis/buck,rmaz/buck,robbertvanginkel/buck,justinmuller/buck,clonetwin26/buck,luiseduardohdbackup/buck,sdwilsh/buck,Heart2009/buck,rmaz/buck,Heart2009/buck,bocon13/buck,tgummerer/buck,rmaz/buck,dushmis/buck,rowillia/buck,romanoid/buck,pwz3n0/buck,k21/buck,SeleniumHQ/buck,shs96c/buck,kageiit/buck,dushmis/buck,robbertvanginkel/buck,Dominator008/buck,k21/buck,zhan-xiong/buck,1yvT0s/buck,stuhood/buck,raviagarwal7/buck,illicitonion/buck,siddhartharay007/buck,Distrotech/buck,grumpyjames/buck,daedric/buck,siddhartharay007/buck,brettwooldridge/buck,shs96c/buck,Addepar/buck,illicitonion/buck,mogers/buck,dpursehouse/buck,zpao/buck,hgl888/buck,rowillia/buck,shybovycha/buck,Dominator008/buck,mogers/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,rmaz/buck,robbertvanginkel/buck,lukw00/buck,nguyentruongtho/buck,neonichu/buck,mogers/buck,k21/buck,brettwooldridge/buck,mnuessler/buck,grumpyjames/buck,Learn-Android-app/buck,vschs007/buck,illicitonion/buck,Dominator008/buck,rowillia/buck,rowillia/buck,raviagarwal7/buck,shybovycha/buck,1yvT0s/buck,1yvT0s/buck,OkBuilds/buck,facebook/buck,pwz3n0/buck,mikekap/buck,nguyentruongtho/buck,neonichu/buck,vschs007/buck,mogers/buck,artiya4u/buck,illicitonion/buck,mikekap/buck,zhan-xiong/buck,daedric/buck,bocon13/buck,mogers/buck,rowillia/buck,justinmuller/buck,mogers/buck,hgl888/buck,Heart2009/buck,mogers/buck,hgl888/buck,brettwooldridge/buck,dsyang/buck,romanoid/buck,grumpyjames/buck,mikekap/buck,rowillia/buck,raviagarwal7/buck,daedric/buck,zhuxiaohao/buck,vschs007/buck,ilya-klyuchnikov/buck,kageiit/buck,facebook/buck,mogers/buck,nguyentruongtho/buck,marcinkwiatkowski/buck,siddhartharay007/buck,shs96c/buck,ilya-klyuchnikov/buck,shs96c/buck,rmaz/buck,OkBuilds/buck,Dominator008/buck,illicitonion/buck,neonichu/buck,artiya4u/buck,rhencke/buck,vschs007/buck,shs96c/buck,bocon13/buck,romanoid/buck,vine/buck,justinmuller/buck,Addepar/buck,darkforestzero/buck,ilya-klyuchnikov/buck,mogers/buck,LegNeato/buck,vschs007/buck,k21/buck,clonetwin26/buck,shybovycha/buck,davido/buck,zhan-xiong/buck,marcinkwiatkowski/buck,justinmuller/buck,MarkRunWu/buck,liuyang-li/buck,mikekap/buck,Heart2009/buck,vine/buck,Learn-Android-app/buck,vschs007/buck,bocon13/buck,romanoid/buck,Heart2009/buck,robbertvanginkel/buck,JoelMarcey/buck,romanoid/buck,marcinkwiatkowski/buck,vine/buck,facebook/buck,neonichu/buck,shybovycha/buck,neonichu/buck,hgl888/buck,hgl888/buck,lukw00/buck,shs96c/buck,Heart2009/buck,pwz3n0/buck,brettwooldridge/buck,tgummerer/buck,sdwilsh/buck,janicduplessis/buck,dpursehouse/buck,zhuxiaohao/buck,tgummerer/buck,daedric/buck,facebook/buck,Distrotech/buck,Addepar/buck,dsyang/buck,vine/buck,rhencke/buck,luiseduardohdbackup/buck,Distrotech/buck,robbertvanginkel/buck,clonetwin26/buck,rhencke/buck,hgl888/buck,MarkRunWu/buck,zhan-xiong/buck,mogers/buck,darkforestzero/buck,darkforestzero/buck,SeleniumHQ/buck,Distrotech/buck,grumpyjames/buck,lukw00/buck,davido/buck,artiya4u/buck,raviagarwal7/buck,luiseduardohdbackup/buck,liuyang-li/buck,SeleniumHQ/buck,artiya4u/buck,hgl888/buck,mnuessler/buck,k21/buck,SeleniumHQ/buck,Distrotech/buck,janicduplessis/buck,davido/buck,janicduplessis/buck,MarkRunWu/buck,dsyang/buck,daedric/buck,LegNeato/buck,OkBuilds/buck,LegNeato/buck,Heart2009/buck,dushmis/buck,ilya-klyuchnikov/buck,bocon13/buck,robbertvanginkel/buck,bocon13/buck,MarkRunWu/buck,tgummerer/buck,dsyang/buck,vine/buck,JoelMarcey/buck,JoelMarcey/buck,JoelMarcey/buck,shs96c/buck,neonichu/buck,JoelMarcey/buck,rhencke/buck,shs96c/buck,davido/buck,zhan-xiong/buck,Dominator008/buck,brettwooldridge/buck,zpao/buck,zhuxiaohao/buck,grumpyjames/buck,JoelMarcey/buck,artiya4u/buck,JoelMarcey/buck,Addepar/buck,LegNeato/buck,shybovycha/buck,mnuessler/buck,tgummerer/buck,grumpyjames/buck,shs96c/buck,raviagarwal7/buck,k21/buck,janicduplessis/buck,raviagarwal7/buck,lukw00/buck,sdwilsh/buck,brettwooldridge/buck,illicitonion/buck,LegNeato/buck,rmaz/buck,darkforestzero/buck,justinmuller/buck,facebook/buck,darkforestzero/buck,luiseduardohdbackup/buck,Dominator008/buck,dsyang/buck,LegNeato/buck,darkforestzero/buck,dsyang/buck,shybovycha/buck,LegNeato/buck,lukw00/buck,kageiit/buck,dsyang/buck,romanoid/buck,Distrotech/buck,MarkRunWu/buck,lukw00/buck,romanoid/buck,Learn-Android-app/buck,mogers/buck,OkBuilds/buck,clonetwin26/buck,JoelMarcey/buck,SeleniumHQ/buck,daedric/buck,lukw00/buck,tgummerer/buck,marcinkwiatkowski/buck,OkBuilds/buck,luiseduardohdbackup/buck,justinmuller/buck,mikekap/buck,raviagarwal7/buck,sdwilsh/buck,janicduplessis/buck,brettwooldridge/buck,bocon13/buck,clonetwin26/buck,mikekap/buck,Addepar/buck,Distrotech/buck,liuyang-li/buck,marcinkwiatkowski/buck,OkBuilds/buck,liuyang-li/buck,rhencke/buck,illicitonion/buck,clonetwin26/buck,ilya-klyuchnikov/buck,vine/buck,kageiit/buck,nguyentruongtho/buck,Dominator008/buck,stuhood/buck,Dominator008/buck,janicduplessis/buck,1yvT0s/buck,rmaz/buck,luiseduardohdbackup/buck,liuyang-li/buck,neonichu/buck,1yvT0s/buck,ilya-klyuchnikov/buck,mnuessler/buck,vschs007/buck,sdwilsh/buck,SeleniumHQ/buck,tgummerer/buck,ilya-klyuchnikov/buck,Heart2009/buck,darkforestzero/buck,facebook/buck,shybovycha/buck,rmaz/buck,pwz3n0/buck,neonichu/buck,Dominator008/buck,Distrotech/buck,daedric/buck,liuyang-li/buck,justinmuller/buck,grumpyjames/buck,romanoid/buck,clonetwin26/buck,ilya-klyuchnikov/buck,mnuessler/buck,dpursehouse/buck,shybovycha/buck,zpao/buck,facebook/buck,daedric/buck,shs96c/buck,darkforestzero/buck,raviagarwal7/buck,siddhartharay007/buck,hgl888/buck,lukw00/buck,pwz3n0/buck,liuyang-li/buck,davido/buck,raviagarwal7/buck,janicduplessis/buck,vschs007/buck,romanoid/buck,robbertvanginkel/buck,justinmuller/buck,justinmuller/buck,janicduplessis/buck,dsyang/buck,zhan-xiong/buck,1yvT0s/buck,raviagarwal7/buck,mikekap/buck,liuyang-li/buck,kageiit/buck,daedric/buck,brettwooldridge/buck,Addepar/buck,k21/buck,brettwooldridge/buck,vschs007/buck,zpao/buck,janicduplessis/buck,mnuessler/buck,zhuxiaohao/buck,zpao/buck,zhuxiaohao/buck,daedric/buck,darkforestzero/buck,brettwooldridge/buck,pwz3n0/buck,shybovycha/buck,zhan-xiong/buck,zhan-xiong/buck,justinmuller/buck,artiya4u/buck,JoelMarcey/buck,SeleniumHQ/buck,vine/buck,Learn-Android-app/buck,brettwooldridge/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,daedric/buck,zpao/buck,hgl888/buck,JoelMarcey/buck,LegNeato/buck,OkBuilds/buck,robbertvanginkel/buck,LegNeato/buck,davido/buck,artiya4u/buck,grumpyjames/buck,nguyentruongtho/buck,tgummerer/buck,artiya4u/buck,daedric/buck,rowillia/buck,illicitonion/buck,darkforestzero/buck,Addepar/buck,dushmis/buck,rhencke/buck,pwz3n0/buck,raviagarwal7/buck,LegNeato/buck,shybovycha/buck,stuhood/buck,dsyang/buck,rowillia/buck,shs96c/buck,sdwilsh/buck,bocon13/buck,mnuessler/buck,sdwilsh/buck,nguyentruongtho/buck,dushmis/buck,dpursehouse/buck,romanoid/buck,vschs007/buck,Dominator008/buck,k21/buck,pwz3n0/buck,illicitonion/buck,bocon13/buck,OkBuilds/buck,clonetwin26/buck,davido/buck,marcinkwiatkowski/buck,Learn-Android-app/buck,MarkRunWu/buck,JoelMarcey/buck,1yvT0s/buck,marcinkwiatkowski/buck,dpursehouse/buck,dsyang/buck,lukw00/buck,rmaz/buck,mnuessler/buck,janicduplessis/buck,ilya-klyuchnikov/buck,shybovycha/buck,shybovycha/buck,davido/buck,stuhood/buck,Learn-Android-app/buck,romanoid/buck,siddhartharay007/buck,dushmis/buck,brettwooldridge/buck,dsyang/buck,marcinkwiatkowski/buck,pwz3n0/buck,davido/buck,SeleniumHQ/buck,rhencke/buck,raviagarwal7/buck,lukw00/buck,romanoid/buck,siddhartharay007/buck,zhuxiaohao/buck,sdwilsh/buck,robbertvanginkel/buck,kageiit/buck,rhencke/buck,zhuxiaohao/buck,1yvT0s/buck,luiseduardohdbackup/buck,ilya-klyuchnikov/buck,illicitonion/buck,SeleniumHQ/buck,dushmis/buck,clonetwin26/buck,ilya-klyuchnikov/buck,k21/buck,davido/buck,davido/buck,SeleniumHQ/buck,marcinkwiatkowski/buck,marcinkwiatkowski/buck,dsyang/buck,dsyang/buck,vine/buck,dpursehouse/buck,rhencke/buck,grumpyjames/buck,stuhood/buck,mikekap/buck,vschs007/buck,mikekap/buck,clonetwin26/buck,rowillia/buck,sdwilsh/buck,grumpyjames/buck,Distrotech/buck,k21/buck,rowillia/buck,shs96c/buck,k21/buck,Learn-Android-app/buck,OkBuilds/buck,liuyang-li/buck,sdwilsh/buck,sdwilsh/buck,MarkRunWu/buck,OkBuilds/buck,illicitonion/buck,stuhood/buck,illicitonion/buck,SeleniumHQ/buck,zhan-xiong/buck,rmaz/buck,illicitonion/buck,marcinkwiatkowski/buck,Dominator008/buck,OkBuilds/buck,stuhood/buck,darkforestzero/buck,kageiit/buck,lukw00/buck,raviagarwal7/buck,siddhartharay007/buck,vine/buck,grumpyjames/buck,Addepar/buck,bocon13/buck,stuhood/buck,OkBuilds/buck,vschs007/buck,SeleniumHQ/buck,dushmis/buck,justinmuller/buck,luiseduardohdbackup/buck,JoelMarcey/buck,Addepar/buck,Addepar/buck,clonetwin26/buck,mikekap/buck,LegNeato/buck,shybovycha/buck,zhan-xiong/buck,k21/buck,robbertvanginkel/buck,artiya4u/buck,bocon13/buck,clonetwin26/buck,darkforestzero/buck,rmaz/buck,brettwooldridge/buck,rhencke/buck,davido/buck,rowillia/buck,Addepar/buck,bocon13/buck,Learn-Android-app/buck,justinmuller/buck,siddhartharay007/buck,k21/buck,artiya4u/buck,Learn-Android-app/buck,rowillia/buck,pwz3n0/buck,zhuxiaohao/buck,Dominator008/buck,ilya-klyuchnikov/buck,dpursehouse/buck,JoelMarcey/buck,dushmis/buck,davido/buck