Don't respond to a buffer ack unless there is an error

Change-Id: I751d387c3ece85574610dc0b8f4542506b509f89
Reviewed-on: http://review.couchbase.org/38993
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>