Project - add editor-level support for detecting name conflicts

Summary: Fixes T4833. I wish there was an elegant way to catch this exception but I think the stack is written such that we really should just do this one-off query here...

Test Plan: from the "create project" link under "edit task" I received a more detailed exception than the report in T4833 post patch. I also tested editing an existing project - yay - and editing an existing project to some other existing project's name - got a nice error dialogue.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4833

Differential Revision: https://secure.phabricator.com/D8834