Add original mail headers to the bottom of error mail

Summary: From IRC, this is sometimes helpful for debugging if there's a mailing list issue or something like that. For example, it can show "To" and "Cc".

Test Plan: Got some email, saw headers in it.

Reviewers: dctrwatson, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8708