check if url returns 404 in header
var url = 'https://mapsengine.google.com/map/embed?mid=zGLD-nMIJQAg.k0gTyNlOPz_Q';
function UrlExists(url) {
var http = new XMLHttpRequest();
http.open('HEAD', url, false);
http.send();
if (http.status != 404)
// do something
else
window.location.reload();
}