swarming_bot: silence IOError during time.sleep()
swarming_bot: silence IOError during time.sleep()

This happens when time.sleep() calls returns with '[Errno 4] Interrupted
function call' which gets converted to an IOError exception, as the host is
shutting down.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug:
Change-Id: I742f7ed1bb325f0d58edaea5586a3490269adfb2
Reviewed-on: https://chromium-review.googlesource.com/765264
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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