ZooKeeperSaslClient#respondToServer should log exception message of SaslException

When upstream like HBase call ZooKeeperSaslClient with security enabled, we sometimes get error in HBase logs like:

SASL authentication failed using login context 'Client'.

This error occures when getting SaslException in ZooKeeperSaslClient#respondToServer :

 catch (SaslException e) {
                LOG.error("SASL authentication failed using login context '" +
                        this.getLoginContext() + "'.");
                saslState = SaslState.FAILED;
                gotLastPacket = true;
  }

This error makes user confused without explicit exception message. So I think we can add exception message to the log.