add SocketOptionError unknown error

[dragonnn]

During debugging (probably will open an issue about it soon) DtlsSocket I did hit the _ => panic!("Unknown error code: {}", errno), in SocketOptionError.
I think it is better to return an Unknown error instead of crashing the whole application.

[diondokter]

Hi, thanks for the PR!

I deliberately left it be a panic. The codes all need to be handled correctly or the socket can get into weird undefined states. The only thing the user could do is panic or close the socket and since I can't force users to close the socket (not ergonomically at least), panic is the only option I have.

And if the panic wasn't there, you might have just accepted it being there if it was easily recoverable and I wouldn't become aware of it and the crate would never handle it properly.
So the panic works, because you're here now 😉

So I'm inclined to keep the panic, because it genuinely should never panic and if it does I need to hear about it.

[dragonnn]

Hmmm I would be probably here too if the DtlsConnect returned an error :D. But I am fine with keeping it as I already found the issue and it clearly my error: #25

[diondokter]

Hmmm I would be probably here too if the DtlsConnect returned an error :D. But I am fine with keeping it as I already found the issue and it clearly my error: #25

Ha good, but sadly not everyone is such a good Samaritan :P