swarming: downgrade ensure_active_slice errors to warnings
swarming: downgrade ensure_active_slice errors to warnings

BUG=1002758
TEST=None

Change-Id: If4cb2e04522a45df491d0b05776dc59611587975
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1828173
Auto-Submit: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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