Function add_failure

Small utility function to add propstat for one failure

Package: davical\caldav
Copyright: Morphoss Ltd - http://www.morphoss.com/
License: GNU GPL v2
Author: Andrew McMillan andrew@mcmillan.net.nz
Located at caldav-PROPPATCH.php

Parameters summary
`unknown_type`	`$type`	$tag
`unknown_type`	`$tag`	$status
`unknown_type`	`$status`	$description
`unknown_type`	`$description = null`	$error_tag
	`$error_tag = null`