Exception used to raise an engine related error. This exception has the
status - status code to send to client
reason a text string giving the reason for the refusal.
ClassMethod new ($status,$reason[,$response])
$status - status code to send to client
$reason - string reason for failure
$response - custom TWiki::Response object to be sent to client. Optional.
All the above fields are accessible from the object in a catch clause
in the usual way e.g.
Generate a summary string. This is mainly for debugging.