gddo-server: only redirect ASCII errors to #43036

When a request 404s, but not due to an invalid character, redirect to
pkg.go.dev instead of #43036, since that isssue isn't relevant.

Change-Id: I241fe3b380a7f7faf6022246eeb33f4cd441f3f4
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/287795
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>