Bring error handling docs up to date

The nice doc updates from @mikeando were making me feel guilty about the error handling docs.

The `docs/error-handlng.md` file dates back to the early error redesign work and doesn't match up with the route we actually took. This PR brings it in line with actual practice both for external users and internal implementors, and notes some of the exceptions and possible bugs.

It's not perfect, but I think it's a lot closer to reality than what was there.