Better handle artifact server unimplemented methods

Background

At the moment, calling an unimplemented method would raise an unhandled NotImplementedError on the artifact server side. Would be better to simply return a gRPC error with an UNIMPLEMENTED status to the client.

Task description

ContentAddressableStorage.BatchUpdateBlobs (implementation in progress #676 (closed)).
ContentAddressableStorage.GetTree (no implementation plan yet).
ByteStream.QueryWriteStatus (no implementation plan yet).

Edited Oct 02, 2018 by Martin Blanchard