raise_404 mechanism for custom templates

[simonw]

Having tried this out I think it does need a raise_404() mechanism - which needs to be smart enough to trigger the default 404 handler without accidentally going into an infinite loop.

Originally posted by @simonw in #944 (comment)

[simonw]

Documentation: https://docs.datasette.io/en/latest/custom_templates.html#returning-404s