We run into this issue in several different waterfalls now:

https://wasm-stat.us/console
https://build.chromium.org/p/client.nacl/waterfall
https://build.chromium.org/p/client.nacl.sdk/waterfall

The problem is that if there is, for example, a python exception
in the annotated_steps in the current stop should really be
marked as red.  Right no all the the steps remain green even
though the top level annotated_steps is red.

We can fix this by doing something like this I think, but it seems
like the kind of thing that the annotated_steps task could handle
automatically:

```
  try:
    return run()
  except:
    traceback.print_exc()
    if buildbot.current_step:
      buildbot.Fail()
    return 1
```