Improve recovery from panel action rendering exceptions, and mark "Changeset" queries as not suitable for panel generation

Summary:
Fixes T13443. When a panel raises an exception during edit action generation, it currently escapes to top level. Instead, catch it more narrowly.

Additionally, mark "ChangesetSearchEngine" as not being a suitable search engine for use in query panels. There's no list view or search URI so it can't generate a sensible panel.

Test Plan:
  - Added a changeset panel to a dashboard.
  - Before: entire dashboard fataled.
  - After: panel fataled narrowly, menu fatals narrowly, dashboard no longer permits creation of another Changeset query panel.

Maniphest Tasks: T13443

Differential Revision: https://secure.phabricator.com/D20902

src/applications/dashboard/engine/PhabricatorDashboardPanelRenderingEngine.php

@@ -329,9 +329,16 @@ final class PhabricatorDashboardPanelRenderingEngine extends Phobject {
     $actions = array();
 
     if ($panel) {
+      try {
         $panel_actions = $panel->newHeaderEditActions(
           $viewer,
           $context_phid);
+      } catch (Exception $ex) {
+        $error_action = id(new PhabricatorActionView())
+          ->setIcon('fa-exclamation-triangle red')
+          ->setName(pht('<Rendering Exception>'));
+        $panel_actions[] = $error_action;
+      }
 
       if ($panel_actions) {
         foreach ($panel_actions as $panel_action) {

src/applications/differential/query/DifferentialChangesetSearchEngine.php

@@ -22,6 +22,10 @@ final class DifferentialChangesetSearchEngine
     return 'PhabricatorDifferentialApplication';
   }
 
+  public function canUseInPanelContext() {
+    return false;
+  }
+
   public function newQuery() {
     $query = id(new DifferentialChangesetQuery());