MB-8572: Always throw {error, Msg}

Change-Id: Ia1f2b3f71bbf5d30f1378fee31d6c8748144189d
Reviewed-on: http://review.couchbase.org/27569
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>