Improve error handling of \Drupal class

The \Drupal class currently assumes that the container is set, and fails in a very unhelpful way when this assumption is not true:

PHP Fatal error: Call to a member function get() on a non-object in /var/www/d8/core/lib/Drupal.php on line 186

The attached patch instead throws an exception, and provides the method being called on \Drupal in the message.