On python3, in grpc._links.service._Kernel._on_service_acceptance_event,
there is a runtime TypeError:

```
_on_service_acceptance_event
    group, method = service_acceptance.method.split('/')[1:3]
TypeError: 'str' does not support the buffer interface
```

It is fixed by using a bytes literal (`b'/'`) instead of a string literal.

This exposed another issue in grpc.beta._server where an exception was being
raised with a bytes literal for a message (a string should be used instead.)