Propagate error from find_cluster_by_name correctly.

Change-Id: I2a98731905b0d22b9bfcb7608e1ecd060a781f35
Reviewed-on: http://review.couchbase.org/36755
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>