gddo-server: do not tee 4xx and 5xx requests

If a request returns a 4xx or 5xx on godoc.org, do not tee the request.

Change-Id: Icf71945699f639627449822a58cd03040b164cfb
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/287233
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Trust: Julie Qiu <julie@golang.org>