Default responder should not console.error 4xx errors

Right now the default handler (last step) will `console.error` when it gets an `err`, otherwise 404. I think that it probably shouldn't `console.error` when the `err` it got was a 4xx error, mainly because a 4xx is supposed to be a client error, of which the server admin probably cannot do anything about. Adding a custom error handler could, of course, let the user log 4xx error all they wanted--this is just about the default behavior. It could probably wait until express 5, though.