Display Web Archives popup on 404 error

This isn't a big deal, but it would be nice to have an option to automatically display a popup with archive options when you arrive at a 404 page or similar. I could see myself forgetting about this extension and giving up on a broken page otherwise. Plus, it would just be a little quicker and more convenient. Here's what I'm imagining:

No files, tests, or entry points are named. Start by locating how the extension detects navigation failures and opens its existing interface, then determine how 404 or similar responses can trigger the proposed archive-options popup. Done should include a defined trigger, an option to enable it, and coverage for the expected behavior.

Written by the indexing model from the issue text.