How to display the 404 error page in Symfony2 dev environment

Question

I want to work on the 404 page from the dev environment. I customize 404 using this file : app/Resources/TwigBundle/views/Exception/error.html.twig

This prod url work correctly : mysite.com/404

But this one mysite.com/app_dev.php/404 throw a NotFoundHttpException and give me the dev debug page.

Is it possible to display the error page instead of debug page ?

UPDATE:

The official documentation has now a chapter about that : Testing Error Pages during Development

Answer 1

To display error pages, in web/app_dev.php change second parameter to false

$kernel = new AppKernel('dev', false);

After testing what you need, change it back.

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UPDATE

Thanks @user2019515 for pointing that out - now (2.3 and up) there's a link to WebfactoryExeptionsBundle in Symfony docs and the method I wrote above should not be used.