Provide nicer string for trying to move a task to its current columns

Summary: Ref T6027. We got a not-very-user-friendly default string before.

Test Plan: Selected "Move", didn't change the dropdown, hit submit. Now, got a nice human-readable description of the issue.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6027

Differential Revision: https://secure.phabricator.com/D15649