YamlFileLoader use assertions instead of exceptions

To me this is won't fix, services.yml files should always be parseable and there is a chance someone has a broken one only in production somehow - we should always notify of this even at runtime, if something fails to load here it is early warning that more things will be broken.

@Spokje also pointed out this file has a note saying it's mostly copied from Symfony, so yeah let's keep them in sync and not deviate.

We might want to make a more general task/plan for following up on 📌 Follow up for "Adding Assertions to Drupal - Test Tools" Fixed .