> It only works when your function returns error of the same type as expression inside 'try!' or '?' macros.

The language has actually thought of this case -- it turns out you can map lower-level errors into your higher-level error type by implementing the `From<E>` trait. E.g.:

    use std::io;
    pub enum MyErr { Io(io::Error), Custom(String) }

    impl From<io::Error> for MyErr {
        fn from(err: io::Error) -> MyErr { MyErr::Io(err) }
    }

    fn do_stuff() -> Result<(), MyErr> {
        let f = try!(File::open(...));
        // ...
    }