Simplify some more older mail error handling code

Summary: Ref T4371. We can reuse more code for this "your stuff is empty" error, now, and benefit from global rate limiting and being able to reply to arbitrary addresses.

Test Plan: Sent valid, empty, and empty-ignored email via `mail_handler.php`, got appropriate actions/errors/states.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4371

Differential Revision: https://secure.phabricator.com/D8701