lib/couchbase.js exports wrong error code for "notStored"

The error code returned by the server when appending/prepending to a non-existing key is CONST.NOT_STORED (18), not CONST.KEY_ENOENT (13).

See attached demonstration:

Error on prepend:

{ [CouchbaseError: Item not stored (did you try to append/prepend to a missing key?)] message: 'Item not stored (did you try to append/prepend to a missing key?)', code: 18 }

Error on append:

{ [CouchbaseError: Item not stored (did you try to append/prepend to a missing key?)] message: 'Item not stored (did you try to append/prepend to a missing key?)', code: 18 }