Issue a proper 404 when trying to edit nonexistent events

Summary: We'll fatal a little later here when trying to call methods. 404 instead.

Test Plan: Visited `/calendar/event/edit/9999999/` or similar.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8591

src/applications/calendar/controller/PhabricatorCalendarEventEditController.php

@@ -47,6 +47,9 @@ final class PhabricatorCalendarEventEditController
             PhabricatorPolicyCapability::CAN_EDIT,
           ))
         ->executeOne();
+      if (!$status) {
+        return new Aphront404Response();
+      }
 
       $end_time->setValue($status->getDateTo());
       $start_time->setValue($status->getDateFrom());