Better error messages in the herald transcript
Better error messages in the herald transcript

Summary:
Always return error() when an exception derails creating diff builds,
which will cause phabricator to show a failed step/the error message in herald.
Use logging.exception so that this shows up in sentry (hopefully with stack
trace info).

Right now, the pre-existing logging.error calls create sentry errors without
stack trace info (so you don't see them when searching for
phabricator_notify_diff.) Not all of these are in try/except blocks, so I can't
just replace them all with logging.exception. If you want to take a minute and
suggest how I rewrite this class for optimal error handling, that would be
great.

Test Plan: .

Reviewers: haoyi, kylec

Reviewed By: kylec

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D135680

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