gocql/gocql — pull request #143 by Zariel (2014-04-08 11:18:05+00:00)
Title: Use better error messages instead of ErrUnavailable

When a session is closed it should not be possible to execute queries,
the error message for this is distinct to a live session which has
not connections.

If a live session has no available connections then indicate that
instead of the generic ErrUnavailable.