[PR] Fix error message z-index
[PR] Fix error message z-index

Summary:
Change CodeMirror error message z-index, so that it always appear on top of navigation bar
Pull Request resolved: https://github.com/facebook/flow/pull/7775

Test Plan:
Before:

{F164165100}

After:

{F164165144}

Reviewed By: gabelevi

Differential Revision: D16056936

Pulled By: panagosg7

fbshipit-source-id: a1735db47df461bbd2b6712d23b2d7cb97d0b153

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