RDMA client/server protocol should return errno

[jti-lanl]

For now, we're just returning success/failure, from the server. But then the client-side ends up picking up some arbitrary errno, when it comes to reporting the failure, which can be misleading. It should be fairly easy (no changes to the protocol, per se) to return e.g. negative-errno for failures, which the receiver can turn into failure plus errno.

This might or might not make much difference for callers of libne proper, but it would make the RDMA test_client more useful.