gs: Add a new SSL error to RetryFilter.

It's been seen that a SSL error can surface possibly due to network
flakes.  This error gets raised to a GSCommandError which can fail a
stage.  The error that is encountered is the following:

  `ssl.SSLError: [Errno 8] _ssl.c:510: EOF occurred in violation of
  protocol`

This commit adds portions of that string to the RetryFilter.

BUG=chromium:653137
BRANCH=None
TEST=python gs_unittest --network

Change-Id: I6c63259e4efdfcfd1f97a854f67e7de95714989b
Signed-off-by: Aseda Aboagye <aaboagye@google.com>
Reviewed-on: https://chromium-review.googlesource.com/399679
Commit-Ready: Aseda Aboagye <aaboagye@chromium.org>
Tested-by: Aseda Aboagye <aaboagye@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
(cherry picked from commit bfd179865c093d65c4c3598b74c90f6d2680687b)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2348397
Tested-by: Mike Frysinger <vapier@chromium.org>
2 files changed