js: Throw GObject.NotImplementedError when requiring overriding

Since version 1.50.0, gjs defines GObject.NotImplementedError for throwing
errors when a "virtual" method that requires a subclass implementation is not
defined.

So use this instead of a generic JS Error in such cases.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/549

js/gdm/batch.js

@@ -124,7 +124,7 @@ var Batch = class extends Task {
     }
 
     process() {
-        throw new Error('Not implemented');
+        throw new GObject.NotImplementedError(`process in ${this.constructor.name}`);
     }
 
     runTask() {