Raise an exception when git is not found
Raise an exception when git is not found

Fixes #531 as such exceptions are ignored in powerline.lib.vcs.guess.
s0undt3ch/powerline,blindFS/powerline,DoctorJellyface/powerline,xxxhycl2010/powerline,cyrixhero/powerline,S0lll0s/powerline,blindFS/powerline,seanfisk/powerline,bezhermoso/powerline,kenrachynski/powerline,firebitsbr/powerline,bezhermoso/powerline,seanfisk/powerline,Luffin/powerline,s0undt3ch/powerline,xfumihiro/powerline,cyrixhero/powerline,junix/powerline,S0lll0s/powerline,Luffin/powerline,wfscheper/powerline,russellb/powerline,QuLogic/powerline,kenrachynski/powerline,blindFS/powerline,firebitsbr/powerline,Liangjianghao/powerline,EricSB/powerline,dragon788/powerline,junix/powerline,darac/powerline,bartvm/powerline,junix/powerline,DoctorJellyface/powerline,prvnkumar/powerline,xxxhycl2010/powerline,areteix/powerline,seanfisk/powerline,Liangjianghao/powerline,prvnkumar/powerline,xxxhycl2010/powerline,darac/powerline,darac/powerline,bezhermoso/powerline,lukw00/powerline,xfumihiro/powerline,bartvm/powerline,s0undt3ch/powerline,IvanAli/powerline,russellb/powerline,russellb/powerline,IvanAli/powerline,xfumihiro/powerline,prvnkumar/powerline,cyrixhero/powerline,dragon788/powerline,wfscheper/powerline,dragon788/powerline,QuLogic/powerline,firebitsbr/powerline,lukw00/powerline,EricSB/powerline,areteix/powerline,bartvm/powerline,lukw00/powerline,IvanAli/powerline,QuLogic/powerline,kenrachynski/powerline,S0lll0s/powerline,wfscheper/powerline,DoctorJellyface/powerline,EricSB/powerline,Luffin/powerline,Liangjianghao/powerline,areteix/powerline