Gracefully handle no git or hg on Windows
Gracefully handle no git or hg on Windows

Summary:
Unfortunately Windows behaves differently if you try to call `create_process` with a non-existent command. This meant the Windows build did not work if you didn't have git or hg in your path.
Closes https://github.com/facebook/flow/pull/2129

Reviewed By: samwgoldman

Differential Revision: D3615169

Pulled By: gabelevi

fbshipit-source-id: 367f1ff4c051578af80ef71e7c54dd25af886101

gabro/flow,jamesgpearce/flow,jdelStrother/flow,AprilArcus/flow,AgentME/flow,samwgoldman/flow,MichaelDeBoey/flow,JonathanUsername/flow,jeffmo/flow,popham/flow,TiddoLangerak/flow,AgentME/flow,ylu1317/flow,samwgoldman/flow,mroch/flow,chenglou/flow,claudiopro/flow,samwgoldman/flow,gabro/flow,nmote/flow,jdelStrother/flow,nmote/flow,samwgoldman/flow,MichaelDeBoey/flow,AgentME/flow,popham/flow,jdelStrother/flow,gabelevi/flow,jeffmo/flow,AprilArcus/flow,JonathanUsername/flow,nmote/flow,gabro/flow,jamesgpearce/flow,MichaelDeBoey/flow,jdelStrother/flow,ForbesLindesay/flow,facebook/flow,popham/flow,nmote/flow,AgentME/flow,AprilArcus/flow,jgrund/flow,claudiopro/flow,gabro/flow,facebook/flow,jeffmo/flow,jamesgpearce/flow,popham/flow,popham/flow,ForbesLindesay/flow,fletcherw/flow,TiddoLangerak/flow,JonathanUsername/flow,AprilArcus/flow,gabro/flow,fletcherw/flow,jgrund/flow,gabelevi/flow,samwgoldman/flow,mroch/flow,AgentME/flow,jamesgpearce/flow,ylu1317/flow,TiddoLangerak/flow,TiddoLangerak/flow,chenglou/flow,chenglou/flow,MichaelDeBoey/flow,facebook/flow,claudiopro/flow,mroch/flow,mroch/flow,samwgoldman/flow,gabelevi/flow,jamesgpearce/flow,gabro/flow,AprilArcus/flow,ForbesLindesay/flow,ylu1317/flow,gabelevi/flow,TiddoLangerak/flow,jdelStrother/flow,facebook/flow,popham/flow,JonathanUsername/flow,jdelStrother/flow,ForbesLindesay/flow,fletcherw/flow,jeffmo/flow,mroch/flow,jeffmo/flow,ylu1317/flow,fletcherw/flow,chenglou/flow,nmote/flow,JonathanUsername/flow,samwgoldman/flow,AprilArcus/flow,jeffmo/flow,gabelevi/flow,jgrund/flow,facebook/flow,facebook/flow,nmote/flow,MichaelDeBoey/flow,mroch/flow,jgrund/flow,claudiopro/flow,gabelevi/flow,JonathanUsername/flow,JonathanUsername/flow,nmote/flow,ylu1317/flow,chenglou/flow,ForbesLindesay/flow,gabelevi/flow,jgrund/flow,facebook/flow,popham/flow,jamesgpearce/flow,claudiopro/flow,mroch/flow,chenglou/flow,jamesgpearce/flow,fletcherw/flow,ForbesLindesay/flow,jgrund/flow,fletcherw/flow