If a page is requested but not found in the set of built-in
pages PageNotFoundHook is called to try to handle the request.
PageNotFoundHook is passed the requested URI and a reference to the
ServerHTTP connection. If it can handle the request, it returns an array
of ($httpcode, $content, @httpheaders), else undef.
PageNotFoundHook sub {return (200, "your HTML content");}