Fix for issue #363: Page loading error method

[martinkou]

The code and test cases are similar to #1122 but this time the message box logic has been encapsulated into a new widget called "notification".

[scottjehl]

Thanks. We're moving this to the feature requests page in the wiki where we can consider it later as a widget. I think we'll need something a bit lighter for 363, like a callback event that you can handle however you'd like. Thanks!