Update span status code setting in gRPC interceptor to reflect changes in the spec

[matej-g]

It seems after open-telemetry/opentelemetry-specification#966, and also taking into consideration changes made to codes pkg (open-telemetry/opentelemetry-go#1214), the span error codes are no longer 1:1 with gRPC errors.

When setting a span status based off the Status from grpc in otelgrpc/interceptor.go, this change should be taken into consideration.

Possibly could also resolve #295

[Aneurysm9]

#391 addresses this to some respect in that it needed to update the span status codes to codes.Error to account for open-telemetry/opentelemetry-go#1214. @matej-g is there anything else you feel needs to be done for this issue, or can it be closed with the v0.13.0 release?

[matej-g]

I think the release addresses the issue completely, this can be closed, I believe together with #295.