Fixed custom error pages outputting headers

If we don't have this method called, then the controller returns an object. From then on, [Symfony converts this object in to a string](https://github.com/symfony/HttpFoundation/blob/master/Response.php#L406). This string is **not just the content of the view**, but the headers as well.

After attaching this method, `handleCustomError()` returns a string and everything works as expected.

modules/system/classes/ErrorHandler.php

@@ -49,7 +49,7 @@ class ErrorHandler extends ErrorHandlerBase
 
         // Route to the CMS error page.
         $controller = new Controller($theme);
-        return $controller->run('/error');
+        return $controller->run('/error')->getContent();
     }
 
     /**