Don't send error/exception mail to unverified addresses

Summary:
Depends on D19017. Fixes T12491. Ref T13053. After SES threw us in the dungeon for sending mail to a spamtrap we changed outbound mail rules to stop sending to unverified addresses, except a small amount of registration mail which we can't avoid.

However, we'll still reply to random inbound messages with a helpful error, even if the sender is unverified.

Instead, only send exception mail back if we know who the sender is.

Test Plan: Processed inbound mail with scripts/mail/mail_handler.php. No more outbound mail for "bad address", etc. Still got outbound mail for "unknown command !quack".

Reviewers: amckinley

Maniphest Tasks: T13053, T12491

Differential Revision: https://secure.phabricator.com/D19018