extensionsService: Log error if preferences dialog failed to open

This is helpful for continuous integration checks to see whether the
preferences dialog of an extension can be opened successfully.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1830>

js/dbusServices/extensions/extensionsService.js

@@ -211,6 +211,7 @@ var ExtensionPrefsDialog = GObject.registerClass({
             this._stack.visible_child = widget;
         } catch (e) {
             this._setError(e);
+            logError(e, 'Failed to open preferences');
         }
     }