Need observe response to indicate a state of not found, rather than return an error response

How can I detect replication state using OBSERVE? Currently it is stops processing keys when some key isn't available and returns result.

Use case:

There are two keys "foo" and "bar" on the cluster with 3 replicas, and "bar" is persisted and replicated, but "foo" isn't available on some replicas.
To check the the state of the keys, the client determines the vbucket ids for active node and replicas, groups keys by nodes and sends them out. If for example the node X must contain some replica of "foo" (missing now) and replica of "bar" it might reply NOT_FOUND code for whole query.

Maybe it should use persisted field and put some code there meaning the key isn't available for given vbucket?