This is needed in the release recipes because there are times when we
want to check that a file is _not_ there yet (i.e. 404 response).

R=agable@chromium.org, dnj@chromium.org

Change-Id: Iedf646d6c73ebf898e90afd2009840b1e5c5b1ab
Reviewed-on: https://chromium-review.googlesource.com/709916Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>