Throw a proper 404 page when accessing a task which does not exist.

src/applications/maniphest/controller/taskdetail/ManiphestTaskDetailController.php

@@ -34,6 +34,9 @@ class ManiphestTaskDetailController extends ManiphestController {
     $priority_map = ManiphestTaskPriority::getTaskPriorityMap();
 
     $task = id(new ManiphestTask())->load($this->id);
+    if (!$task) {
+      return new Aphront404Response();
+    }
 
     $transactions = id(new ManiphestTransaction())->loadAllWhere(
       'taskID = %d',

src/applications/maniphest/controller/taskdetail/__init__.php

@@ -6,6 +6,7 @@
 
 
 
+phutil_require_module('phabricator', 'aphront/response/404');
 phutil_require_module('phabricator', 'applications/differential/parser/markup');
 phutil_require_module('phabricator', 'applications/maniphest/constants/priority');
 phutil_require_module('phabricator', 'applications/maniphest/constants/status');