Visit this endpoint for a `MediaWiki:` interface message that's overridden on the local wiki, e.g. https://fr.wikipedia.org/w/rest.php/v1/page/MediaWiki:Editcheck-config.json/html
Observe the following error: `{"message":"Error: exception of type InvalidArgumentException","httpCode":500,"httpReason":"Internal Server Error"}`
Note that the top-level page API for the message shows its source correctly (https://fr.wikipedia.org/w/rest.php/v1/page/MediaWiki:Editcheck-config.json), unlike in T349677 where that endpoint fails for non-overridden messages.
A wiki with the message //not// overridden (https://nah.wikipedia.org/w/rest.php/v1/page/Huiquimedia%3AEditcheck-config.json/with_html) does show the `with_html` endpoint correctly, though not the base one.
Note that this works fine for messages that have wikitext content, e.g. https://fr.wikipedia.org/w/rest.php/v1/page/MediaWiki:Abusefilter/html