@Override public final InputStream getErrorStream() {
    try {
      HttpEngine response = getResponse();
      if (HttpEngine.hasBody(response.getResponse())
          && response.getResponse().code() >= HTTP_BAD_REQUEST) {
        return response.getResponse().body().byteStream();
      }
      return null;
    } catch (IOException e) {
      return null;
    }
  }
Returns an input stream from the server in the case of error such as the
requested file (txt, htm, html) is not found on the remote server.
gateway/platforms/servlet/src/main/java/io/apiman/gateway/platforms/servlet/connectors/ok/HttpURLConnectionImpl.java