Convert DebugContext::failScreenshots after step to Behat 3.

The actual `DebugContext::failScreenshots` after step method does not work on Behat 3 installation: the `@AfterStep @javascript` annotation on [line 42](https://github.com/Behatch/contexts/blob/1ae2062899a7b43d4b2dae6b78e097e980916cc8/src/Context/DebugContext.php#L42) do not work any more.

I've fix it. But I haven't find any way to get the `ScenarioNode` instance form an `AfterStepScope`. So I've write the, not that nice, method `getScenario`. If anyone has a better way to do, I take.