fail to other pool member based on http status code

Just check the HTTP::status value on the response and issue a HTTP::redirect if it matches your criteria.

This iRule should do the trick.

when HTTP_RESPONSE {
  if { [HTTP::status] >= 400 } {
    HTTP::redirect "http://www.someotherdomain.com/"
 }
}

I would just watch out for 401 (authentication required) status codes. You would likely not want to redirect the user in this case.

Another option exists if all you want to do is present an error message. You can use the HTTP::respond command to send custom HTML back to the user directly from within the iRule. This would eliminate the need for a separate backend server just to service errors. If you have application logic you need to run on the redirect, then this won't really help you.

-Joe