src: Replace class "errorbox" per T300306
src: Replace class "errorbox" per T300306

Class `errorbox` will be removed in a future verison of MediaWiki.
Replace it with `mw-message-box mw-message-box-error`.

Closes https://github.com/Krinkle/mw-gadget-rtrc/pull/105.

Krinkle/mw-gadget-rtrc