MB-100 log status of the proxy in case of unhandled cast

Change-Id: I622e1cc672af0c4f1e5dd8317868a0d913e90231
Reviewed-on: http://review.couchbase.org/34903
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>