[FLINK-977] Improve error reporting for failed TM connection.

I also added a log message when the JobManager removes a TaskManager because it disconnected.
A user had an issue recently with the TM registration behavior: http://apache-flink-incubator-user-mailing-list-archive.2336050.n4.nabble.com/Task-manager-has-not-been-registered-td26.html