One thing you may be missing is that the `Result::Err` in your return type must implement `From` for the `Result::Err` type where you're using the `?` operator. That's already covered if those `Err` types are identical (`From<T> for T`), otherwise you need to implement the conversion.

    impl From<QuestionError> for ReturnError {
        fn from(error: QuestionError) -> ReturnError {
            // TODO
            unimplemented!()
        }
    }