Bugfix: handle error case gracefully.

in queryport/cluster_client.go

Change-Id: Ib927c528034ccd9dd99c8cd648c7b64a12343ea2
Reviewed-on: http://review.couchbase.org/43348
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>