If a linearizable read in a sharded cluster targets multiple shards (we do not disallow this even though linearizability cannot be guaranteed on more than 1 document), one of which is a replica set updated to 3.4 and another is only on version 3.2, the linearizable read command might succeed on the 3.4 set and return linearizable read not supported on the 3.2 set. We need to make sure mongos knows how to respond to this situation.