Missing .exp file error should indicate the file's intended name
Missing .exp file error should indicate the file's intended name

Summary:
This has been driving me nuts for the last few days (not because of the goof itself, but because I authored the goof).  When a test directory's .exp file is missing, the error message looks like "Missing tests/some_test/.exp or .flowconfig...."  This is slightly unclear to a new dev and it looks like hell.
Closes https://github.com/facebook/flow/pull/1837

Differential Revision: D3352271

Pulled By: mroch

fbshipit-source-id: 04ec7e402b52848cd9fe36fa757d885aecfd8250

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