[DOC beta] Assert if template for {{render}} not found.

Added an assertion if the template name provided to `{{render}}` was not 
found.

Two tests were previously not working properly, and have been fixed. They
registered templates as `"blog/post"`, which is never looked up with
`{{render}}` (as any slashes in the name are translated to a period before
this lookup happens).

Resolves #3926.