Implement 404 for requests toward non-implemented routes

Currently, when hitting a route which contains another method, users receive a 405 with a JSON message about the issue.

To align ourselves with PHP aurweb's style, we need to handle these routes.

1. For any requests toward routes which do not have implemented handlers, return 404.
2. For any requests toward routes which do not implemented the HTTP method requested, return a 405 in Arch-theme and explain.
3. Particularly for /{pkgname}, where /{pkgname} matches no other route, we should give package suggestions as is done in https://aur.archlinux.org/makedeb (on live).