Steps to reproduce:

1. Go to https://en.wikipedia.org/w/index.php?title=Main_Page&action=edit (not logged-in and private/incognito mode)
2. Choose visual editor from the editor option box
3. Make some changes and attempt to save it

Actual: An error message that says docserver-http: HTTP 404, which isn't very helpful, is shown

Expected: Do something that makes more sense and more user-friendly.