Cardinality violations should use error code “unimplemented”

The gRPC docs for error codes state that both client and server should use the `unimplemented` code for cardinality violations. See table at the bottom of this doc (you can search for “cardinality violation” in the doc): https://grpc.github.io/grpc/core/md_doc_statuscodes.html.

A cardinality violation is when a response stream contains an incorrect number of messages. Specifically, when the response stream for a unary RPC contains zero messages with an OK status or more than one message. (The same goes for client-stream RPCs, which also expect a unary response, but this package does not support client-streaming.)

The client in this package returns an `unknown` error in this situation instead of `unimplemented`.