Use return instead of try_files
Use return instead of try_files

Use the nginx rewrite module's `return` directive instead of
`try_files`.

It's simpler and `try_files` is confusing: I'm pretty sure the
`not_found.html` in `try_files /proxy/not_found.html =404;` doesn't
exist in either Via 3 or NGINX and doesn't do anything.

hypothesis/via,hypothesis/via,hypothesis/via