style1.proto

syntax = "proto3";

message Error {
  string message = 1;
  int32 error_code = 20;
}

message Application {
  string id = 1;
  string type = 2;
}

message CreateApplicationRequest {
  string type = 1;
}

message CreateApplicationResponse {
  oneof SuccessOrError {
    Application application = 1;
    Error error = 20;
  };
}

service Server {
  // Sends a greeting
  rpc CreateApplication (CreateApplicationRequest) returns (CreateApplicationResponse) {}
}

style2.proto

syntax = "proto3";


message Application {
  string id = 1;
  string type = 2;
  string error = 20
}

service Server {
  rpc CreateApplication (Application) returns (Application) {}
}

Didn't know about oneof thats pretty cool!

I think prefer style2.proto. Seems more similar to signature from most Go functions. The CreateApplication function returns an error (or nil for error):

Client:

req :=  &pb.Request{...}
res, err := pb.CreateApplication(ctx, req)

if err != nil {
  // error handling
  return
}

Service:

func (s *server) CreateApplication(ctx context.Context, req *pb.Request) (*pb.Result, error) {
  // ...   
  if someErrorCondition == true {
    return &pb.Result{}, errors.New("Some error message")
  }

  return &pb.Result{...}, nil
}