Use the ops._private.harness.ActionFailed object

[tonyandrewmeyer]

@benhoyt and I decided that rather than monkey(patch) about with having an ActionFailed object in both Harness and Scenario and then trying to merge them when using Scenario via ops.testing, it would be better to just have the Harness object support a state attribute (always None when using with Harness).

This means that the object can be removed from Scenario (I guess re-exporting the name for backwards compatibility) and the ops._private.harness one used instead.