zookeeper: exit program on unrecoverable error

This adds panic() where sheep receives unrecoverable error from
ZooKeeper.  It's not good to abort the program on error, but much
better than destroying membership information on each node.  It is
future work to remove those panic().

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>