Stop using File_key.Builtins during Try Flow libdef init
Stop using File_key.Builtins during Try Flow libdef init

Summary:
See D36103697, which makes a similar change for regular server init.

This diff also removes some unused error handling code. We don't report libdef
errors in Try Flow. There should not be libdef errors in Try Flow.

Changelog: [internal]

Reviewed By: mroch

Differential Revision: D36105060

fbshipit-source-id: efe1e6bfdd4ba7fe4685c88ad95fed9438fd6128

facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow