Resolve "disable "Add overlay" button or improve error message `403` when number of custom layout is greater equal `CUSTOM_LAYOUTS`"

Closes #611 (closed)

frontend-js/src/main/js/gui/AddOverlayDialog.js

@@ -292,7 +292,7 @@ AddOverlayDialog.prototype.open = function () {
               var errorMessage = JSON.parse(error.content);
               GuiConnector.alert("Problematic input: <br/>" + errorMessage.reason);
             } else if (error instanceof SecurityError) {
-              GuiConnector.alert("You cannot add more overlays");
+              GuiConnector.alert("You cannot add more overlays, Custom Overlay limit is reached.");
             } else {
               GuiConnector.alert(error);
             }