'resources/src/mediawiki.skinning/messageBoxes.css' classes are from early times in MediaWiki core, where CSS class architecture wasn't yet a thing.
The classes should be turned into
- messagebox => mw-message-box
- errorbox => mw-error-box
- warningbox => mw-warning-box
Without strong opinion if *-box is the right naming scheme, we're using *-container in a related, but slightly different context.
There would be also the possibility to use mw-error and mw-error--inline assuming that user notice boxes are the default way of exposing them.