Previously these were recorded in the PackageInfo, but not
reported to the user's error handler (types.Config.Error),
which is typically what prints them.
Minor subtlety: that function must now be able to handle error
values that are not of type types.Error.
+ Test (and renamed it).
LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/121290043
Alan Donovan