It's such a common error, we should try and give a friendly error
message when it occurs.

Closes #1603.

The config was picking the wrong directory so the custom test templates
were not used during tests.

Notes

- Fix missing i18n load tags
- Remove mixed tab/space indentation
- Add two settings tests

This avoids conflicts with other libraries that use the same trick for
extending templates.

Also split them into unit and functional tests.