Throw a more helpful error for bad Differential actions

Summary: Ref T7611. This should let us figure out the root cause, hopefully.

Test Plan: iiam

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7611

Differential Revision: https://secure.phabricator.com/D12124

src/applications/differential/editor/DifferentialTransactionEditor.php

@@ -249,6 +249,11 @@ final class DifferentialTransactionEditor
           case DifferentialAction::ACTION_CLAIM:
             $object->setAuthorPHID($this->getActingAsPHID());
             return;
+          default:
+            throw new Exception(
+              pht(
+                'Differential action "%s" is not a valid action!',
+                $xaction->getNewValue()));
         }
         break;
     }