notificationDaemon: Use registered D-Bus error

Instead of returning ad-hoc errors, register a custom error domain
and return appropriate GLib.Errors.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3159>

js/misc/dbusErrors.js

@@ -34,3 +34,10 @@ const modalDialogErrorNames = [
 export const ModalDialogErrors =
     registerErrorDomain('ModalDialog', modalDialogErrorNames);
 export const ModalDialogError = createErrorEnum(modalDialogErrorNames);
+
+const notificationErrorNames = [
+    'InvalidApp',
+];
+export const NotificationErrors =
+    registerErrorDomain('Notifications', notificationErrorNames, 'org.gtk');
+export const NotificationError = createErrorEnum(notificationErrorNames);