Fix a potential crash when looking at the runner summary.
Fix a potential crash when looking at the runner summary.

I'm not sure what the root cuase of the crash is, so log an
error if it is seem again.

R=maruel@chromium.org
BUG=

Review URL: https://codereview.appspot.com/30020043

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