Decode value only for successful codes

Change-Id: Ib0b9e163f9bd956cc3ba379c9060d6a73658ad7f
Reviewed-on: http://review.couchbase.org/28966
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>