Make 404 Not Found reporting more consistent

[wandernauta]

This changes the following:

• Some paths had not-found handling that would not set the response status, just a human-readable message or empty string; make this consistent with the other paths (see Een niet-bestaand document geeft HTTP 200 met een foutmelding als inhoud #143).
• Some paths had 404 response bodies, but these were actually ignored in favor of the response body generated by the error handler. These are now removed.
• The original error page would describe 404 errors as a malfunction ('Storing'). That wording is now slightly changed in specifically the 404 case. The other suggestions to users are unchanged.
• The original error page would fail to load correctly for some paths. An error page for /foo/bar/baz would look for /foo/bar/pico.min.css, which would fail, and then the font and links on the page would be broken. This sets a sensible base URL with the HTML base element.

Before:

After:

Where paths had specific logging for the not-found case, this is kept.

[wandernauta]

Build failure seems unrelated, see #149.