gRPC: How to get multiple return values in Java client with Go server

I have a protocol buffer file:

syntax = "proto3";
package v1api;
option java_multiple_files = true;
option java_package = "myApp.v1";
option java_outer_classname = "V1";

service API {
    rpc Login(LoginRequest) returns (LoginResponse)
}

message LoginRequest {
    int pin = 1
}

message LoginResponse {
    string token = 1
}

I have my server written in Go (a language that can return multiple values), and my client being an Android application.

When I use this protoBuf to generate Go code for server, it is something like:

...
func (c *aPIClient) Login(ctx context.Context, in *LoginRequest, opts ...grpc.CallOption) (*LoginResponse, error) {
    out := new(LoginResponse)
    err := grpc.Invoke(ctx, "/v1api.API/Login", in, out, c.cc, opts...)
    if err != nil {
        return nil, err
    }
    return out, nil
}
...

Notice that there are two return values, (*LoginResponse, error).

Now, when I use this protoBuf to generate Java code for my Android side, I get something like:

public myApp.v1.LoginResponse login(myApp.v1.LoginRequest request) {
    return blockingUnaryCall(getChannel(), METHOD_LOGIN, getCallOptions(), request);
}

Notice that, here, there is just one return value like Java allows, myApp.v1.LoginResponse.

My question is, in case of an error returned by the server (something like: return nil, err), how will I get that second return value in my Android side.