404, 500, etc. handlers

[mlrawlings]

404 could easily be done by providing a catchall route:

exports.path = /.*/;
exports.handler = (req, res) => {
   res.send(404, 'Page Not Found');
}

But it might be nice to have something a bit more built in, and especially for error handlers. It would also allow outputing a 404.html and similar for GitHub pages.