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

src/applications/transactions/storage/PhabricatorApplicationTransaction.php

@@ -727,6 +727,10 @@ abstract class PhabricatorApplicationTransaction
         return pht('This object is already in that space.');
       case PhabricatorTransactions::TYPE_EDGE:
         return pht('Edges already exist; transaction has no effect.');
+      case PhabricatorTransactions::TYPE_COLUMNS:
+        return pht(
+          'You have not moved this object to any columns it is not '.
+          'already in.');
     }
 
     return pht(