client does not receive gRPC error

We tried to use grpc-web with [grpcwebproxy](https://github.com/improbable-eng/grpc-web/blob/master/go/grpcwebproxy) and ran into an issue that the client could not receive gPRC error.

1. When server returns an error, our callback gets `{err: null, response: }`.

[`GatewayClientBase.prototype.rpcCall`](https://github.com/grpc/grpc-web/blob/master/javascript/net/grpc/web/gatewayclientbase.js#L57) calls the callback on stream "data" and error "status" events. However the stream error "status" event never fires because it tries to look up [`grpc-status`](https://github.com/grpc/grpc-web/blob/master/javascript/net/grpc/web/grpcwebclientreadablestream.js#L160) while the trailers body contains [`Grpc-Status`](https://github.com/grpc/grpc-go/blob/master/internal/transport/handler_server.go#L263).

We found a related issue https://github.com/improbable-eng/grpc-web/issues/228.

Which component was supposed to handle this case mismatch?

2. Suppose the error "status" issue is fixed. When a gRPC error happens, the client first receives `{err: null, response: }`, then `{err: , response: null}`. The client has to deal with the `` response everywhere, unable to tell easily it was a failure other than calling its getters returning `undefined`. What's recommended pattern for a client to differentiate success vs. error?