Handle termination of coordinator.

Catch exceptions when trying to tell the coordinator about a failed TCP
connection.  This is visible when running the test suite; when the
application shuts down and open TCP connections terminate,
ringleader_connection instances try to notify the coordinator of a
tcp_closed message, which has already terminated.
Merge pull request #5 from cmeiklejohn/feature/csm/termination-fsm

Handle termination of coordinator.