First, after seeing the 404 website you mentioned, nobody ever wants to do that.
The ErrorHandler, as explained in the manual page, only allows you to specify a file in disk. If you want to redirect to another website, try this UrlToolkit rule:
UrlToolkit {
ToolkitID = 404page
RequestURI exists Return
Match .* Redirect http://www.thebest404pageever.com/
}