Suggest using aio.channel when throwing error in async contexts.

[DeltaEvo]

See https://github.com/google/avatar/blob/main/avatar/pandora_client.py#L136 for context.

We should change the error message to something like Trying to use the synchronous gRPC channel from asynchronous code. Use "aio.channel" instead of "channel" to get a "grpc.aio.Channel"

[DeltaEvo]

Or maybe add a note https://docs.python.org/3/library/exceptions.html#BaseException.add_note

https://peps.python.org/pep-0678/