code: func ConvertResponse(re *roundtrip.Response, err error) (*roundtrip.Response, error) {
  	if err != nil {
  		if uerr, ok := err.(*url.Error); ok && uerr != nil && uerr.Err != nil {
  			return nil, trace.ConnectionProblem(uerr.Err, uerr.Error())
  		}
  		return nil, trace.ConvertSystemError(err)
  	}
  	return re, trace.ReadError(re.Code(), re.Bytes())
  }
docs: // ConvertResponse converts http error to internal error type
  // based on HTTP response code and HTTP body contents