Convert AuthError to Error<AuthErrorKind>

[nepalez]

This PR is a part of error specialization initiated by the https://github.com/nats-io/nats.rs/pull/1047

See the initial discussion of the change in this draft PR (to be removed later, after processing the feedback by @Jarema and creating smaller PR-s from its commits).

This part is devoted to convertion of the AuthError into a specialization of the error::Error<T> generic. The error messages are moved to the implementation of its kind AuthErrorKind.