Identify exceptions that should cause NTLM fallback in xplat NegotiateStream

[vijaykota]

This is with reference to this PR comment. Currently, for any exceptions in the SPNEGO codepath, the xplat implementation falls back to NTLM. We need to check if there are specific exceptions we should target.

[vijaykota]

Closing this since we are trying out GSS-NTLMSSP instead. With that we expect the fallback to happen automatically inside the MIT GSSAPI library.