Need to revisit error handling

[MarcusLongmuir]

Currently the error checking and mapping to gRPC codes is quite basic.

This needs revisiting to conform with https://github.com/grpc/grpc/blob/master/doc/statuscodes.md

[roboslone]

It seems to me to be a pretty important issue. I believe there's currently no way to process errors like Unauthenticated.

@MarcusLongmuir could you direct me to a relevant sources? I think I can help.