LPS-40421 More sophisticated error from the TunnelingServetAuthVerifier

Hey Tomáš,

This is a staging fix, although it involves the AuthVerifier framework. I saw you wrote the interface itself, so I've figured out that you might know more about that part than me.

The staging part has been reviewed by me, that is fine, could you please review specifically the AuthVerifier code pieces?

Thanks,

Máté