Make returned code from zk-flock more informatively.

Now zk-flock return 0 on:
    * cannot acquire lock
    * child end work
and 1 on:
    * any errors with watchers and zookeeper
    * receiving SIGTERM
    * configuration error
    * When unable to start child process
    * Lost lock event
All information about reasons is in the log.
If SIGCHLD is received - log child returning code