Setting HTTP_PROXY or HTTPS_PROXY environment variables will cause git cl
to fail.

The behavior of an unreachable server behind proxy is different from a direct
connection, it won't throw a httplib2.ServerNotFoundError error,
so catch httplib2.Httplib2Error instead.

Bug: 1050380
Change-Id: I32e2220483de1fd6222c7fd6063039fb86277ef0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2050947Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>