So far it's alright, and it does what it says on the tin. However, it is probably too eager to prompt the user to check the 404 page. On many occasions where the page did load, its modal window pops in my face asking me to check out any archived versions.

Worse still, the popup is a modal window that blocks out the rest of the page and cannot actually be exited out of without reloading the whole page.

Personally, I think it would be fine if there was no pop-up at all. Simply have the user access the toolbar button.