The logic for saving the list of packages was not always
preferring to keep error messages around correctly.
The missed error led to an internal consistency failure later.

Fixes #17119.

Change-Id: I9723b5d2518c25e2cac5249e6a7b907be95b521c
Reviewed-on: https://go-review.googlesource.com/31812
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>