Improve validation errors for changing task priorities

Summary:
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.

Test Plan:
Before:

{F5007964}

After:

{F5007965}

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