change return type of check step to match merge step, simplify
change return type of check step to match merge step, simplify

Summary: Merge returns an assoc list mapping files to their errors. On the other hand, check returns a map. This diff makes check return an assoc list instead. This enables some code reuse in future diffs.

Reviewed By: samwgoldman

Differential Revision: D15462573

fbshipit-source-id: 2d475a9a154a491e56e92a6ac63ec20f5b202942

samwgoldman/flow,gabelevi/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,gabelevi/flow,gabelevi/flow,gabelevi/flow,samwgoldman/flow,facebook/flow,mroch/flow,facebook/flow,samwgoldman/flow,mroch/flow,facebook/flow,gabelevi/flow,gabelevi/flow,mroch/flow,samwgoldman/flow,mroch/flow,facebook/flow,mroch/flow,samwgoldman/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,samwgoldman/flow