Removes state machine complication and duplication out of Server &
Transport and puts it into the Framer instead (where it's nicely
tested).

Also, for testing, start tracking the reason for errors. Later we'll
use it in GOAWAY frames' debug data too.

Change-Id: Ic933654a33edb62b4432c28fe09f7bfdb6f9b334
Reviewed-on: https://go-review.googlesource.com/18101Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>