Consider what an error message to a non-existing register should return

[bruno-f-cruz]

Related to harp-tech/core.pico#79

In summary, the current specs suggest an error bit should be set if users attempt to write/read from a non-existing register. However there doesnt seem to specify what the contents of the message are expected to be. I can see at least a couple of options:

1. Return the same message contents as were written with the error bit set
2. Return a message with the dtype of the request but 0 length payload

I am leaning towards 2. since it reduces bytes on the wire AND ensures that it is obvious that whatever was returned must not be processed downstream.