Symfony 2.5: Custom 404 error page on ResourceNotFound

Question

I want to implement logic that shows a custom 404 error page on a Symfony environment when there's no route found and a NotFoundHttpException has been thrown.

I've followed the cookbook solution line by line, also number of other sources, including StackOverflow and even trying to handle it via listener and I just can't make it work.

No matter what url I load on stage/prod it always returns 500 Server error. I've added several error pages in:

/app/Resources/TwigBundle/views/Exception/
    - error.html.twig
    - error404.html.twig
    - error500.html.twig
    - exception.html.twig
    - exception404.html.twig
    - exception500.html.twig

just to try and catch both errors and/or exceptions but there's no change. Still Server error 500 shown when trying to load http://mystage.env/non-existing-url. It never really loads any error/exception content.

The app_stage.php and app_prod.php both use false on debugger

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

I have a feeling I am missing some obvious configuration property in my parameters.yml or routing.yml or both, but I have no idea what. So far in my config.yml file part of the Twig configuration is:

# Twig Configuration
twig:
    exception_controller:  twig.controller.exception:showAction
    debug:  "%kernel.debug%"

When I dump the template, that needs to be loaded here:

# vendor/symfony/symfony/src/Symfony/Bundle/TwigBundle/Controller/ExceptionController.php

 public function showAction(
     Request $request, 
     FlattenException $exception, 
     DebugLoggerInterface $logger = null
 )
 {
    var_dump((string) $this->findTemplate(
        $request, 
        $request->getRequestFormat(), 
        $code, 
        $this->debug
    )); die;

    #...
 }

it shows the correct and expected template: string 'TwigBundle:Exception:error404.html.twig' Yet it doesn't load it, but returns 500 Server error.

For the record, I also tried using the WebfactoryExceptionsBundle, but the result was the same.

EDIT

I removed the Twig configuration line for the exception_controller, as suggested in the comments. This is what the app/logs/stage.log says:

request.ERROR: 
Uncaught PHP Exception Symfony\Component\HttpKernel\Exception\NotFoundHttpException: 
"No route found for "GET /nonexisting"" at 
/var/www/html/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/EventListener/RouterListener.php line 145 
{
 "exception":"[object] (
  Symfony\\Component\\HttpKernel\\Exception\\NotFoundHttpException(code: 0): 
    No route found for \"GET /nonexisting\" at 
    /var/www/html/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/EventListener/RouterListener.php:145, 
    Symfony\\Component\\Routing\\Exception\\ResourceNotFoundException(code: 0):  at 
    /var/www/html/app/cache/stage/appStageUrlMatcher.php:1018
 )"
} []

request.ERROR: 
Exception thrown when handling an exception (
 Symfony\Component\Security\Core\Exception\AuthenticationCredentialsNotFoundException: 
 The security context contains no authentication token. 
 One possible reason may be that there is no firewall configured for this URL.
) 
{
 "exception":"[object] (
   Symfony\\Component\\HttpKernel\\Exception\\NotFoundHttpException(code: 0): 
   No route found for \"GET /nonexisting\" at 
   /var/www/html/vendor/symfony/symfony/src/Symfony/Component/HttpKernel/EventListener/RouterListener.php:145, 
   Symfony\\Component\\Routing\\Exception\\ResourceNotFoundException(code: 0):  at 
   /var/www/html/app/cache/stage/appStageUrlMatcher.php:1018
  )"
 } []

Same errors are logged whether I'm successfully logged or not.