[SPARK-2583] ConnectionManager error reporting

This patch modifies the ConnectionManager so that error messages are sent in reply when uncaught exceptions occur during message processing.  This prevents message senders from hanging while waiting for an acknowledgment if the remote message processing failed.

This is an updated version of @sarutak's PR, #1490.  The main change is to use Futures / Promises to signal errors.
You'll notice that I removed a bunch of fields here.  `attempted` was never read anywhere, and `acked` implied `ackMessage != None`.