Have you, too, hacked up some solution to test the different error pages your customers will see for Error 404, 500 etc.?
Symfony 2.6 comes with a great new Feature for previewing error pages.
All you need to do to profit from this is to import a routing file in app/config/routing_dev.yml like this:
_errors: resource: "@TwigBundle/Resources/... ... config/routing/errors.xml" prefix: /_error
To preview the error pages, open up:
The Format Parameter is optional. With it you can not only check HTML error responses but also the ones for JSON and XML.