minor #6224 Note to create a service if you extend ExceptionController (pamuche)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #6224).

Discussion
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Note to create a service if you extend ExceptionController

I had some trouble to get the custom exception handling with inheritance of ExceptionController working, because I hadn't a clue to make a service out of the ExceptionController of the TwigBundle.

Commits
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fff23f7 Note to create a service if you extend ExceptionController

Author: wouterj

cookbook/controller/error_pages.rst

@@ -217,7 +217,25 @@ will be passed two parameters:
 Instead of creating a new exception controller from scratch you can, of course,
 also extend the default :class:`Symfony\\Bundle\\TwigBundle\\Controller\\ExceptionController`.
 In that case, you might want to override one or both of the ``showAction()`` and
-``findTemplate()`` methods. The latter one locates the template to be used.
+``findTemplate()`` methods. The latter one locates the template to be used. 
+
+.. note::
+  
+  In case of extending the :class:`Symfony\\Bundle\\TwigBundle\\Controller\\ExceptionController` 
+  you may configure a service to pass the Twig environment and the ``debug`` flag to the constructor. 
+  
+  .. configuration-block::
+  
+      .. code-block:: yaml
+  
+          # app/config/config.yml
+          services:
+            app.extension_controller:
+              class: AppBundle\CustomExceptionController
+              arguments: [ "@twig", "%kernel.debug%" ]
+          
+          twig:
+              exception_controller:  app.extension_controller:showAction
 
 .. _use-kernel-exception-event: