When authentication fails (bad credentials, non-premium account, etc), librespot panics! instead of returning a proper Result::Err.

I see authenticate and Session::connect both return a io::Error instead of a custom result/error type. Is that intentional? If not, would you be open to a PR introducing a dedicated error type? (I suppose this would require some discussion in terms of implementation and potentially breaking changes).

(by the way, thank you very much for the outstanding work involved in this project!)