Improve validation errors for changing task priorities
Ref T12124. Currently, Conduit provides a fairly rough error message if you provide an invalid priority.
Instead, provide a more tailored message. Also, block !!unknown!! except from web edits.
Also, changed a priority to 999 in the database, edited it with the normal web UI form, it let me make the edit without being forced to adjust the priority.
Reviewers: amckinley, chad
Reviewed By: amckinley
Maniphest Tasks: T12124
Differential Revision: https://secure.phabricator.com/D18135