Differentiate authentication failures from other session setup failures

We provide a programmatic way of establishing netconf sessions, but for failure paths we do not differentiate between the various modes of failure.

Specifically it is hard to discern when a connection fails due to authentication issue, and downstreams have to rely on parsing our error message to find that out.

Provide well-defined exception types to handle the various root causes of failures based on their source domain (transport, auth, etc.)