Show a nice 404 page when database objects are not found

[matrixbot]

This issue was originally created by @sandhose at matrix-org/matrix-authentication-service#2880.

We have an okay-ish fallback page as a general 404, but in many places, we lookup objects by their IDs, and if we can't find them, we just display an 'unexpected error' page.

We should instead in those places also give the standard 404 page, maybe tweaked a little to say that the route is fine, but the object wasn't found?