extensionSystem: Fix error reporting in lg / ListExtensionErrors()

Commit c99e8eb29 removed this line probably accidentally, as no
effort has been made to replace consumers in lookingGlass and
shellDBus.

https://bugzilla.gnome.org/show_bug.cgi?id=690173

js/ui/extensionSystem.js

@@ -129,6 +129,7 @@ function logExtensionError(uuid, error) {
     extension.state = ExtensionState.ERROR;
     if (!extension.errors)
         extension.errors = [];
+    extension.errors.push(message);
 
     log('Extension "%s" had error: %s'.format(uuid, message));
     _signals.emit('extension-state-changed', { uuid: uuid,