Prevent CiviCRM Group Roles Sync from trying to sync to nonexistent groups

I traced a very strange bug in a client's site to civicrm_group_roles. It turns out they had a rule syncing a Drupal role to a CiviCRM group which had been deleted. The result is that an API_Exception is raised, causing a more-or-less unrelated database transaction to be rolled back.

The civicrm_group_roles_add_remove_groups() function already does an API lookup to determine whether or not the group is a smart group; I've modified it a bit so we can validate that the group still exists.