Steps to reproduce:
- Put English Wikipedia in read only mode with: https://gerrit.wikimedia.org/r/#/c/346251/3/wmf-config/db-eqiad.php
- Edit any page (I used what I think is the source version of the visual editor, but I would assume it works similary in all options (the message is nice on the old source editor)
- Hit save
- Ugly json is shown, instead of pretty explanation that mediawiki is in read only, and the message why.
Probably a bit of json parsing the error + CSS would be more than enough.