lib: Introduce Controller::httpNotFound()

Many actions require loading specific objects from database or any other storage.
If the object isn't found, Controller::httpNotFound($message) should be used
for immediately returning w/ HTTP 404.

library/Icinga/Web/Controller.php

@@ -3,6 +3,7 @@
 
 namespace Icinga\Web;
 
+use Zend_Controller_Action_Exception;
 use Icinga\Data\Sortable;
 use Icinga\Data\QueryInterface;
 use Icinga\Web\Controller\ModuleActionController;
@@ -49,6 +50,18 @@ class Controller extends ModuleActionController
         }
     }
 
+    /**
+     * Immediately respond w/ HTTP 404
+     *
+     * @param   $message
+     *
+     * @throws  Zend_Controller_Action_Exception
+     */
+    public function httpNotFound($message)
+    {
+        throw new Zend_Controller_Action_Exception($message, 404);
+    }
+
     /**
      * Create a SortBox widget and apply its sort rules on the given query
      *