Some HTTP proxies may return an HTTP error such as 503 when DNS
resolution fails for GceAuthenticator._INFO_URL
(http://metadata.google.internal), which causes e.g. git-cl to throw an
exception rather than falling back to checking for .netrc or
.gitcookies.

Make GceAuthenticator._get() also consider an httplib2 proxy error a
sign that the code is not running in GCE.

Change-Id: Icc094829f9b044176be44e51f3cad49d4a374f25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2982611
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>