log message if back-channel has stray message.

fixed cbq-scripts.

Change-Id: I74445d60807c70ce24ae99cda164e6923f575384
Reviewed-on: http://review.couchbase.org/43678
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>