Add a warning when there is no matching start tag for an end tag

[liamrharwood]

The following syntax is allowed by Jinjava:

{% if true %}
foo
{% endif %}
{% endif %}

This causes no issues in isolation, but can result in mismatched end tags when inserted in the middle of a template such as in a generated module_attribute tag. Adding a warning will help make this danger more apparent and prevent cases such as this from causing problems.