Calling `step` with invalid action - no error

[worikgh]

For the CartPole-v0 example passing a invalid action does not result in a error

            let _state = match env.step(vec![0.03], false) {
                Ok(s) => {
                    eprintln!("Step succeeded.  state: '{:?}'", s);
                    if s.done {
                        break;
                    }
                },
                Err(e) => panic!("step failed.  Err: '{}'", e),
            };

The step succeeded branch is taken.