[parser] recover gracefully if expected .tree.json file is malformed
[parser] recover gracefully if expected .tree.json file is malformed

Summary:
when I'm rebasing and get merge conflicts, I often leave the conflict
markers and re-record the expected output.

it was failing because <<<<<< is not valid JSON and we raised on JSON parse
errors. now, we fail the test but print the parse error. `-r` is able to
re-record now that it doesn't crash.

Reviewed By: samwgoldman

Differential Revision: D5705310

fbshipit-source-id: c13724a8c775ded54bf54d0c7882214e34858286

ylu1317/flow,samwgoldman/flow,ylu1317/flow,samwgoldman/flow,facebook/flow,TiddoLangerak/flow,claudiopro/flow,claudiopro/flow,TiddoLangerak/flow,jamesgpearce/flow,mroch/flow,facebook/flow,nmote/flow,jamesgpearce/flow,mroch/flow,facebook/flow,popham/flow,nmote/flow,jamesgpearce/flow,AgentME/flow,MichaelDeBoey/flow,popham/flow,ylu1317/flow,gabelevi/flow,samwgoldman/flow,claudiopro/flow,JonathanUsername/flow,jamesgpearce/flow,jamesgpearce/flow,mroch/flow,mroch/flow,jamesgpearce/flow,samwgoldman/flow,gabelevi/flow,facebook/flow,ylu1317/flow,gabelevi/flow,AgentME/flow,mroch/flow,AgentME/flow,JonathanUsername/flow,popham/flow,samwgoldman/flow,jamesgpearce/flow,facebook/flow,popham/flow,JonathanUsername/flow,nmote/flow,facebook/flow,nmote/flow,popham/flow,popham/flow,MichaelDeBoey/flow,MichaelDeBoey/flow,MichaelDeBoey/flow,nmote/flow,samwgoldman/flow,nmote/flow,JonathanUsername/flow,samwgoldman/flow,gabelevi/flow,gabelevi/flow,facebook/flow,AgentME/flow,nmote/flow,TiddoLangerak/flow,gabelevi/flow,JonathanUsername/flow,ylu1317/flow,gabelevi/flow,mroch/flow,popham/flow,AgentME/flow,claudiopro/flow,JonathanUsername/flow,claudiopro/flow,mroch/flow,TiddoLangerak/flow,MichaelDeBoey/flow,JonathanUsername/flow,TiddoLangerak/flow