Make missing mercurial modules fatal
Make missing mercurial modules fatal

Summary:
Move the exception to the exception key, add a user-friendly message. Raise the
Diagnostic message level to 'fatal', which should tell Buck to shut down the
parser.

Test Plan:
Set project.glob_handler = mercurial, parser.python_interpreter to a Python
binary with no access to mercurial, run buck build.

There is a bug where a final diagnostic before closing down the parser is
ignored entirely; I'll follow up on this with a bug report.

CI

Reviewed By: Coneko

fbshipit-source-id: 9b06389

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