Fix EZP-22513: Exception not thrown when twig template does not exist

https://jira.ez.no/browse/EZP-22513

## Description
We use `Twig_Loader_Chain` as loader, where `Twig_Loader_Filesystem` is first checked, `Twig_Loader_String` then.
When a template fails to be loaded by `Twig_Loader_Filesystem` (method `exists()`), `Twig_Loader_String` is then checked. The problem is that `Twig_Loader_String` always returns true since passed *template name* is supposed to be the template itself.