It is possible to create arbitrary error pages for responses
generated when a http_reply term is thrown. Currently this is only
supported for status 403 (authentication required). To do this,
instead of throwing
http_reply(authorise(Term), , Key), where Key
is an arbitrary term relating to the page you want to generate. You must
then also define a clause of the multifile predicate
phrase(page([ title('401 Authorization Required') ], [ h1('Authorization Required'), p(['This server could not verify that you ', 'are authorized to access the document ', 'requested. Either you supplied the wrong ', 'credentials (e.g., bad password), or your ', 'browser doesn\'t understand how to supply ', 'the credentials required.' ]), \address ]), CustomHTML).