minor #14531 Throw Exception if twig variable is not found (ybenhssaien)

This PR was submitted for the 5.1 branch but it was merged into the 4.4 branch instead.

Discussion
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Throw Exception if twig variable is not found

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

![image](https://user-images.githubusercontent.com/7301643/98787347-aeba2700-23ff-11eb-9957-4ac56ff4e6ca.png)

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Commits
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3547418 Throw Exception if twig variable is not found

Author: javiereguiluz

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