fix: point to 404 page for 4xx responses

[mefellows]

Because we're professionals:

404 page could do with some uplift, but it's not terrible:

Note, this should still work with the new website upgrade, assuming there is a 404/index.html page there at the end of it. If not, we just need to do the update.

[TimothyJones]

Needs more https://http.cat/