Emit errors from NativeZk.

This patch emits errors when they occur on the ZooKeeper connection.  For instance the following are example log messages when errors occur:

Connectivity lost for established session:

2014-05-02 15:49:02,864:17781:ZOO_ERROR@handle_socket_error_msg@1643: Socket [192.168.0.61:2181] zk retcode=-7, errno=60(Operation timed out): connection to 192.168.0.61:2181 timed out (exceeded timeout by 1ms)
2014-05-02 15:49:02,864:17781:ZOO_ERROR@yield@213: yield:zookeeper_interest returned error: -7 - operation timeout

ZooKeeper not listening, not accepting clients:

2014-05-02 15:53:27,837:17812:ZOO_ERROR@handle_socket_error_msg@1723: Socket [192.168.0.61:2181] zk retcode=-4, errno=61(Connection refused): failed while receiving a server response
2014-05-02 15:53:27,837:17812:ZOO_ERROR@zk_io_cb@257: yield:zookeeper_process returned error: -4 - connection loss

2014-05-02 15:53:32,838:17812:ZOO_ERROR@handle_socket_error_msg@1699: Socket [192.168.0.61:2181] zk retcode=-4, errno=61(Connection refused): server refused to accept the client
2014-05-02 15:53:32,838:17812:ZOO_ERROR@zk_io_cb@257: yield:zookeeper_process returned error: -4 - connection loss


Currently, it is not possible to get notified of these conditions to implement graceful handling.  This patch gives the application visibility into connection errors, so they can be handled.