Fix an issue with error handling when no mailers are available

Summary:
Ref T13164. See PHI785. See D19546. I think I didn't test the updated error messaging here entirely properly, since I have some tasks in queue which error out here ("Missing argument 1 to newMailers(...)").

This is an error condition already, but we want to get through this call so we can raise a tailored message.

Test Plan: Tasks which errored out here now succeed. This condition is only reachable if you misconfigure things in the first place.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19572