Relay `not found` to user when using getFromReplica

Currently getFromReplica(id) and family ignore

not found

responses from replica nodes. The current method for checking whether a document exists and getting it mean sending a request to each replica node separately(possibly asynchronously) and checking whether the returned doc exists. This means that a user has to couple their code to their server configuration, resulting in a code change if they change the number of replicas for a bucket. The use case for this is where every set is down with waitforreplica and so it is known that if a doc doesn't exist on a replica node then it doesn't exist at all.

It would be useful to allow a DoesNotExistException or something of the sort to be thrown when a `not found` response is returned as the result of a getFromReplica(id) or getFromReplica(id, ReplicaMode.All).