Add ProtoStatus example

[JamesNK]

Example is under examples/Error

[JamesNK]

It feels ok.

My main feedback is there are a lot of namespaces when everything is used together. For example, on the server:

using Google.Protobuf.WellKnownTypes;
using Google.Rpc;
using Greet;
using Grpc.Core;
using Grpc.StatusProto;

I think we should consider moving the extension methods to an existing namespace to reduce the count by one. e.g. move them from Grpc.StatusProto to Grpc.Core or Google.Rpc.

IMO Grpc.Core is a better choice because this is gRPC-focused functionality and that namespace should always be present because of RpcException and ServerCallContext.