Make incoming mail handling more robust / correct

Summary:

• Properly handle when no mail headers at all can be parsed
• Properly handle when mail headers can be parsed but no subject line can be found

EXCEPTION: (RuntimeException) Undefined index: subject

Closes T15769

Test Plan: See T15769

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: 20after4, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15769

Differential Revision: https://we.phorge.it/D25565