Also:
- replaced remaining panics with Fatal calls
- more comments

Fixes #15572.

Change-Id: Ifb27e80b66700f5692a84078764a1e928d4b310d
Reviewed-on: https://go-review.googlesource.com/22935
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>