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

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6732

Differential Revision: https://secure.phabricator.com/D11303