Use successbox/errorbox instead of bolded text on revision delete form

Use successbox/errorbox instead of bolded text on revision delete form

Same idea as I596cbdc2.

* Avoid style markup in i18n messages. Use successbox instead of
  <strong> inside a message in a span.
* Consistency with rest of MediaWiki (e.g. Special:Preferences).
* Make error look like an error. It was previously output in a plain
  paragraph with no styling or other indication.

Change-Id: Ia2f8d56edbd72632d02c6738841340b28cc6d773