Fixes as suggested by GH reviewers
Make createResponse method protected

The docs (http://symfony.com/doc/current/cookbook/controller/error_pages.html) explicitly
mentioned subclassing the controller and overriding the showAction method to pass
additional variables to the template.

The variables are now defined in createResponse(), so I'd like to keep the possibility
to change them. The new testErrorPageAction() method might even work in these cases.