Merge pull request #2079 from WouterJ/issue_2078

[2.1] Added caution about using is_granted in error pages

Author: weaverryan

cookbook/controller/error_pages.rst

@@ -50,6 +50,13 @@ end-user, create a new template located at
     </body>
     </html>
 
+.. caution::
+
+    You **must not** use ``is_granted`` in your error pages, because the
+    router runs before the firewall. If the router throws an exception (for
+    instance, when the route does not match), the error page is not behind a
+    firewall and ``app.user`` is not defined.
+
 .. tip::
 
     If you're not familiar with Twig, don't worry. Twig is a simple, powerful