extensionSystem: Add missing return value

_callExtensionInit() should return whether the extension was initialized
successfully or not, but the early return added in commit 2a9e065cfb
doesn't.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/715

js/ui/extensionSystem.js

@@ -305,7 +305,7 @@ var ExtensionManager = class {
 
     _callExtensionInit(uuid) {
         if (!Main.sessionMode.allowExtensions)
-            return;
+            return false;
 
         let extension = this.lookup(uuid);
         if (!extension)