Fix parallel_execution which would pass the wrong argument in case of error.
Fix parallel_execution which would pass the wrong argument in case of error.

This can be triggerd by running run_swarming_tests_on_swarming.py without
passing the -I flag.

R=csharp@chromium.org
BUG=

Review URL: https://codereview.appspot.com/123540043

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