Add comment to failing_step to indicate that it raises an exception.
Add comment to failing_step to indicate that it raises an exception.

Change-Id: I77d4001b3e347c55689496e4527bcaf8a17efd04
Reviewed-on: https://chromium-review.googlesource.com/1229237
Commit-Queue: Erik Chen <b51d12b71fa2f8cb17021720ad5ee62fe09eb9e9@chromium.org>
Reviewed-by: Stephen Martinis <f3fbe4c4218d2421400bf0bd07366865fd7a0c7e@chromium.org>

luci/recipes-py,luci/recipes-py