gs should retry on SSL read-operation-timed-out error.

Set "ssl.SSLError: ('The read operation timed out',)" as one
resumable error message.

BUG=chromium:642986
TEST=unit_test

Change-Id: I1b683d72ebe534ca60ae694aede15d91ad85c327
Reviewed-on: https://chromium-review.googlesource.com/399960
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Ningning Xia <nxia@chromium.org>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
(cherry picked from commit 14b255f36c4ca13f551ccca09ba3942df02e37cc)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2348685
Tested-by: Mike Frysinger <vapier@chromium.org>
2 files changed