cmd/go/internal/web: minor api cleanup

- rename PasswordRedacted to Redacted
- move URL into Response in redacted form, remove from Get result list
- add Response.Err to construct non-200 errors
  (otherwise GetBytes is not just a wrapper)
- make 404/410 errors satisfy Is(err, os.ErrNotExist)

Change-Id: Id15899c1e3dfd30cffb1a75ba79a9a1999913258
Reviewed-on: https://go-review.googlesource.com/c/go/+/173717


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>