Error handling

The error type is tskit::error::TskitError.

This type implements Display and may thus be printed via:

let x = match operation() {
    Ok(y) => y,
    Err(e) => panic("{}", e);
};

The enum variant TskitError::ErrorCode represents integer return values from tskit-c. When printed via Display, the detailed error message is fetched. When printed via Debug, the numerical error code is shown.