cmd/go: in 'go list -m', ignore "not found" errors loading updates

"Not found" and "no matching version" errors usually indicate the user
is offline or the proxy doesn't have a version of go.mod that could
provide retractions. 'go list -m -u' should still succeed.

We should still report unclassified errors though. Previously, we
reported most errors loading retractions but did not report errors
loading updates. This change makes those operations more consistent.

Fixes #45305

Change-Id: I2f23a566c9481bc7ff229a177f39d78f6a8aae77
Reviewed-on: https://go-review.googlesource.com/c/go/+/306572


Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>