Throw Exception if twig variable is not found

An exception of type `Twig\Error\RuntimeError` is thrown when a variable is not found and `strict_variables` is enabled

Author: ybenhssaien

templates.rst

@@ -164,7 +164,7 @@ in the following order:
 #. ``$foo->getBar()`` (object and *getter* method);
 #. ``$foo->isBar()`` (object and *isser* method);
 #. ``$foo->hasBar()`` (object and *hasser* method);
-#. If none of the above exists, use ``null``.
+#. If none of the above exists, use ``null`` or throw a ``Twig\Error\RuntimeError`` exception if ``strict_variables`` config is enabled.
 
 This allows to evolve your application code without having to change the
 template code (you can start with array variables for the application proof of