Differential - return a better response for validation error cases

Summary: Fixes T6989. Basically return a nice dialogue like we do for "NoEffect" transactions. This is a little prettier than the other dialogue was. Also, stop adding TYPE_EDGE as a transaction type as we end up having it 2x, which then makes the error get validated 2x.

Test Plan: tried to add myself as a reviewer and got a nice error message.

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6989

Differential Revision: https://secure.phabricator.com/D11448