Letgo is not called after a failed example

Pull request contains a failing test (sorry for the brief copy paste job - but I think it illustrates the issue).

I noticed in one of my scripts that the `letgo` wasn't being executed if the test failed, this can be traced back to `MaintainerInterface::teardown` not called for the failing example in `ExampleRunner`.

I am not sure if this is intentional, or a bug?