Prosebox, now with less "The dog ate my entry!" in Site Updates

I've seen a couple of entries today about entries being eaten by Prosebox. Prosebox doesn't actually decide to mess with you and randomly eat entries - most likely, you either thought you hit the Publish button and instead hit the Preview button, which doesn't really save anything. Or you pressed your browser's hotkey that takes you Back in your browser history.

Either way, it leads to grief and frustration. So I added a little notification box that warns you if you try to leave the entry page without saving. Hopefully this will help!