code: func HTTPCode(e error) int {
  	if e == nil {
  		return 200
  	}
  	code, _ := Value(e, httpCode).(int)
  	if code == 0 {
  		return 500
  	}
  	return code
  }
docs: // HTTPCode converts an error to an http status code.  All errors
  // map to 500, unless the error has an http code attached.
  // If e is nil, returns 200.