/**
   * Validate the response is in expected envlope form with <result> content.
   *
   * @param response response
   * @return Envelope if format is correct and there is no error
   * @throws CentralDispatcherException if the format is incorrect, or the Envelope indicates an
   *     error response.
   */
  private Envelope validateResponse(final WebserviceResponse response)
      throws CentralDispatcherException {
    if (null == response) {
      throw new CentralDispatcherServerRequestException("Response was null");
    }

    if (null != response.getResponseMessage()) {
      logger.info("Response: " + response.getResponseMessage());
    }
    final Document resultDoc = response.getResultDoc();
    if (null == resultDoc) {
      throw new CentralDispatcherServerRequestException(
          "Response content unexpectedly empty. "
              + (response.getResponseMessage() != null ? response.getResponseMessage() : ""));
    }
    try {
      logger.debug(serialize(resultDoc));
    } catch (IOException e) {
      logger.debug("ioexception serializing result doc", e);
    }

    if (!"result".equals(resultDoc.getRootElement().getName())) {
      throw new CentralDispatcherServerRequestException(
          "Response had unexpected content: " + resultDoc);
    }
    final Envelope envelope = new Envelope(response.getResultDoc());
    if (envelope.isErrorResult()) {
      final StringBuffer sb = new StringBuffer();
      final StringBuffer buffer = envelope.errorMessages();
      if (sb.length() > 0) {
        sb.append(buffer);
      } else {
        sb.append("Server reported an error");
        if (null != response.getResponseMessage()) {
          sb.append(": ").append(response.getResponseMessage());
        }
      }
      if (null != response.getResponseMessage()) {
        logger.error("Server reported an error: " + response.getResponseMessage());
      } else {
        logger.error("Server reported an error.");
      }
      throw new CentralDispatcherFailureResponseException(sb.toString());
    }
    return envelope;
  }