MB-100: added ability to http-respond via exception

Change-Id: I9356a03794baba2f15b31df692c35ef8b11682ce
Reviewed-on: http://review.couchbase.org/30151
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>