code: public boolean isHttpError() {
  		if (isRevisitDigest() && (getDuplicatePayload() != null)) {
  			return getDuplicatePayload().isHttpError();
  		}
  		String httpCode = getHttpCode();
  		return (httpCode.startsWith("4") || httpCode.startsWith("5"));
  	}
docs: {@code true} if HTTP response code is either {@code 4xx} or {@code 5xx}.
  @return