R1:cf0bf342555b

Allow MetaMTA adapters to indicate that a mail is permanently undeliverable

Summary: Currently, adapters can only fail mail temporarily. Allow them to indicate a permanent failure by throwing a special exception.

Test Plan: Added and ran unit tests.

Reviewers: wez, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6847