log unexpected rest errors in menelaus_web_remote_clusters

Change-Id: I2e6ccb3ac7535f648f9cc2ca91836dc258927337
Reviewed-on: http://review.couchbase.org/32374
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>