error-pages.txt
My site uses <stack> and runs on <host>. Missing pages and errors show
the host's default pages.

Set up:
- A 404 page that matches the site, says what happened, and offers a way
  forward: home, search, popular pages.
- A 500 page that works even when the app itself is broken, so plain HTML
  with no dependencies.
- Both returning the correct status codes, not 200.
- Where this host lets me configure them.

Then tell me how to test each one on the live host.

Check the status code, not just the look. A friendly 404 page served with status 200 tells search engines every broken link is a real page.