code: public static SendStatus error(final int code, final String message)
     {
        return new SendError(code, message) {
           @Override
           public String toString()
           {
              return "SendStatus.error(" + code + ", \"" + message + "\")";
           }
        };
     }
docs: Create an {@link Operation} that will send an HTTP error code and message to the browser, then call
  {@link HttpInboundServletRewrite#abort()}