In the google_javascript/wagmp_map_1.js file, find the following code:
GEvent.addListener(directions, "error", function() {
map.setCenter(wagmp_map_1_obj.points[0], 13);
map.addOverlay(wagmp_map_1_obj.markers[0]);
});
you can add an alert statement to this to show an error
GEvent.addListener(directions, "error", function() {
if(fromAddress.street != '') {
alert('Invalid Address');
}
map.setCenter(wagmp_map_1_obj.points[0], 13);
map.addOverlay(wagmp_map_1_obj.markers[0]);
});