Make LKGBC logic handle when swarming_bot.zip disapeared under the scene.
Make LKGBC logic handle when swarming_bot.zip disapeared under the scene.

This was observed once on a bot. Instead of copying the missing file back, it
threw an exception at os.stat() and rebooted, leaving the host idle.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1293243003 .

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