correctly output errors of remote_clusters_info POST

Otherwise we'll crash at json formatting

Change-Id: I7f949d224ac2692e35ebb34d9659128477a7271b
Reviewed-on: http://review.couchbase.org/30348
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>