Instead of passing the `bookName` and `bookTitle` parameters to
`Response::build_404()`, `withTaskbarInfo()` is applied to its result
when needed. Note, that in `InternalServer::handle_raw()`
`withTaskbarInfo()` was not utilized since the results of the `/raw`
endpoint are not supposed to be decorated with a taskbar.
Now Response::build_404() takes the URL instead of the entire
RequestContext object. An empty url suppresses the
The requested URL "url" was not found on this server.
part of the error text.