404ErrorImages adds an "error image" to any page that has not been created on a wiki.
Note: only works with articles. There may eventually be one for Special: pages that don't exist, however.
This script works well with a customized version of MediaWiki:Noarticletext, but note: on the CSS, the height: attribute FORCES a height for the div. This is necessary for the div to work correctly with Darwin; but also, keep in mind that your div needs to stay that width, so in the MW page, don't place anything too terribly large or obnoxious.