@Override
  public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
    saslPromise.tryFailure(cause);
  }
@param saslPromise {@code true} if success, {@code false} if server tells us to fallback to
                   simple.
hbase-client/src/main/java/org/apache/hadoop/hbase/security/NettyHBaseSaslRpcClientHandler.java