Non-200 status codes should have default templates

[hoelzro]

Currently, when you submit a request that results in a 404, 403, or most other >= 400 status codes, you get a page with no content, leaving it up to the browser to inform you that "this link is broken" or something. I think we should add default templates for various status codes, with a configuration option to turn them off in case an admin wants to use the webserver to serve such pages.