Implement std::fmt::Display and std::error::Error for exposed error types

[lexika979]

This change allows users of raknet to use eyre and anyhow to propagate the errors upwards using the ? operator:

async fn smth() -> eyre::Result<()> {
    // These work now:
    let mut listener = Listener::bind("0.0.0.0:1234").await?;
    listener.start().await?;

    Ok(())
}

[john-bv]

Looks good, i'll look more in depth later today!