Also catch ValueError in message parsing.

Summary: This will let us diagnose T612.

Test Plan: None.

Reviewers: khamidou

Differential Revision: https://review.inboxapp.com/D751
Remove RETRY_FAIL_CLASSES in mailsync.

For better or for worse, we really have to keep retrying on things like
transient IntegrityErrors due to race condition, or strange errors raised in
message parsing.