code: protected void sendError(HttpServletResponse response, int guacamoleStatusCode,
              int guacamoleHttpCode, String message)
              throws ServletException {
  
          try {
  
              // If response not committed, send error code and message
              if (!response.isCommitted()) {
                  response.addHeader("Guacamole-Status-Code", Integer.toString(guacamoleStatusCode));
                  response.addHeader("Guacamole-Error-Message", message);
                  response.sendError(guacamoleHttpCode);
              }
  
          }
          catch (IOException ioe) {
  
              // If unable to send error at all due to I/O problems,
              // rethrow as servlet exception
              throw new ServletException(ioe);
  
          }
  
      }
docs: Sends an error on the given HTTP response using the information within
  the given GuacamoleStatus.
  
  @param response
  The HTTP response to use to send the error.
  
  @param guacamoleStatusCode
  The GuacamoleStatus code to send.
  
  @param guacamoleHttpCode
  The numeric HTTP code to send.
  
  @param message
  The human-readable error message to send.
  
  @throws ServletException
  If an error prevents sending of the error code.