ENG-4617 Catch SessionExpired in leader elector

It is possible that the leader elector callback runs on a dead
session. A dead session is shutting down ZK. Don't crash on
this exception. If the session is dead, a next viable leader
elector will run anyway.
Merge pull request #498 from VoltDB/eng-4617-testLeaderElector

ENG-4617 Catch SessionExpired in leader elector
yeah, okay.  r=izzy