Add a log message in case a reaped task is not returned to the user.
Add a log message in case a reaped task is not returned to the user.

We currently have no visibility on this case, it's important to know if it is
happening at all. If it's happening, I'll have to re-think the current workflow.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.appspot.com/174250044

pombreda/swarming,madecoste/swarming,madecoste/swarming,pombreda/swarming,luci/luci-py,luci/luci-py,pombreda/swarming,luci/luci-py,pombreda/swarming,madecoste/swarming,luci/luci-py,madecoste/swarming