[1.6.x] Merge pull request #1580 from ianawilson/ticket_16502

Fixed #16502 -- Fixed a TemplateDoesNotExist error that should be an ImproperlyConfigured.

Assistance on the patch from #jambronrose.

Backport of 9b2dc12b8332389d1bfb9e83123a088a084a6a47 from master.