Clearer error message

imp/lib/Prefs/Special/Acl.php

@@ -113,7 +113,7 @@ public function display(Horde_Core_Prefs_Ui $ui)
 
             $view->new_user = $new_user;
         } catch (Horde_Auth_Exception $e) {
-            // No-op
+            $notification->push('Could not authenticate as admin user to obtain ACLs. Perhaps your admin configuration is incorrect in config/backends.local.php?', 'horde.warning');
         } catch (Horde_Exception $e) {
             if (!($e->getPrevious() instanceof IMP_Exception)) {
                 $notification->push($e);