comments on post-authn interceptor flow

[Tom Zeller]

Phew. After being confused about attempted vs intermediate in the
subject c14n flow when copying to the post-authn flow, I think I
understand now, and wrote some flow tests to make sure.

I think I've got it right now in that the post-authn flow turns any
non-proceed event into an error by calling 'HandleError' in the
calling (profile) flow.

There's enough commonality in the subject c14n and profile interceptor
classes that there probably should be some re-use, I just didn't
understand the details until I worked it through.