In the UK at least, there's no such thing as 'attempted manslaughter'. That's just a type of assault. There was no physical harm here, really, so what degree of assault then depends on the intent of the defendant.
I don't know French law but in England this would likely just be a battery charge, unless you can somehow prove that the guy who threw it definitely intended to cause actual or grevious bodily harm to Van der Poel, which I doubt. I imagine France is similar in that to be charged with 'attempting' you need to show intent, not just recklessness, as it isn't a 'completed' crime.