There's a huge difference between "Server Not Found" and a 404 error. The first means that it couldn't find an IP address for the name, the server is not responding, or there's no web server there. The second means that the web server is all functioning properly but the page you asked for doesn't exist on the server. These are always pages generated by the server. It just gets a different response code than the normal 200 "everything's okay" code.

Which is not to say that it wouldn't be useful in both cases, but implying that they're the same thing is wrong.