chore: the gorums codec should return more informative error messages

See issue #212. The error message is useless:
```
rpc error: code = Internal desc = grpc: failed to unmarshal the received message: proto: not found
```
It comes from this line:
```go
desc, err := protoregistry.GlobalFiles.FindDescriptorByName(protoreflect.FullName(msg.Metadata.GetMethod()))
```
We should add a more informative error message; I have code ready to go (will open a PR tomorrow).
