Check crash file exists before reading
Check crash file exists before reading

Summary:
File watcher gives deletion events as well, so this is needed to avoid
attempting to read nonexistent files.

We're getting ENOENT errors from this code, and I suspect this is the reason.

Reviewed By: nikoant

Differential Revision: D19329894

fbshipit-source-id: 46734ea17874e1448d67cc086b363e0abdf07258

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper