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