Maniphest - fix corner case where no-op subscriber change had wrong error
Summary: Fixes T6732. Fix is to stop trying to catch the error in the controller and let the editor do its job.
Test Plan: tried to add an existing subscriber and got an error message about how that wouldn't do anything
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6732
Differential Revision: https://secure.phabricator.com/D11303