Map errors to custom messages

	    error_map message_url http_status ...

	http_status ... is a list of HTTP status codes or Squid error
	messages.

	Use in accelerators to substitute the error messages returned
	by servers with other custom errors.

	    error_map http://your.server/error/404.shtml 404

	Requests for error messages is a GET request for the configured
	URL with the following special headers

	    X-Error-Status:	The received HTTP status code (i.e. 404)
	    X-Request-URI:	The requested URI where the error occurred

	In Addition the following headers are forwarded from the client
	request:

	    User-Agent, Cookie, X-Forwarded-For, Via, Authorization,
	    Accept, Referer

	And the following headers from the server reply:

	    Server, Via, Location, Content-Location

	The reply returned to the client will carry the original HTTP
	headers from the real error message, but with the reply body
	of the configured error message.