tweak exception style to work with python-3

BUG=None
TEST=`./run_tests` passed

Change-Id: I02456eaa72eff2dceab9a2d6040bf46dd6f32aa9
Reviewed-on: https://chromium-review.googlesource.com/173728
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
23 files changed