Don't retry IMAP syncs on MailsyncError, ValueError, or AttributeError.

Summary:
These are always totally fatal errors that we throw manually, or
programming errors, so we shouldn't retry. Retrying just adds extra log spam.

Test Plan: ./runtests & manual verification by throwing errors under the retry

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D244