Return NOT_STORED for append/prepend if key does not exist

Change-Id: Ibacc6ab0038e532433fb8a643491c80cc2b70ee4
Reviewed-on: http://review.couchbase.org/27497
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>