client: retry when 404
client: retry when 404

This is for https://crbug.com/1206639#c18

GAE somehow returns 404 even for existing path.

Bug: 1206639
Change-Id: I08639b70cc93bf70b5ffe067a77ed2b1ec04e4ee
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3113428
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py