Send rpc error in akka grpc

In scalapb grpc, when we wanted to send rpc error, return Future.failed. In akka grpc when I do that, encounter this exception:

Aborting tcp connection to /127.0.0.1:58880 because of upstream failure

For future reference this then lead to an issue and a PR: https://github.com/akka/akka-grpc/issues/510