R1:644f179dd27a

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…