R1:d9cdbdb9fae9

When we fail to process mail, tell the user about it

Summary:
Ref T4371. Ref T4699. Fixes T3994.

Currently, we're very conservative about sending errors back to users. A concern I had about this was that mistakes could lead to email loops, massive amounts of email spam, etc. Because of this, I was pretty hesitant about replying to email with more email when I wrote this stuff.

However, this was a long time ago. We now have Message-ID deduplication, "X-Phabricator-Sent-This-Mail", generally better mail…