be careful of received mail object in error cases too

Summary:
we're tracking down a fatal caused by the mandatory
attachments array parameter change in the reply handler and
yo dawg, heard you like to fatal in your fatal.

Test Plan:
internal test scenario with xmail -> phabricator
triggered manually.

Reviewers: nh, vrana, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D4005