Fix 'Warning: Proper idle not in action_queue'

The queue action's action_queue_find_idle() function would wrongly announce an error when it could not find an idle action in its user's action queue because they were using a drinks machine with a use_object action. This commit makes this function announce a warning instead.
Merge pull request #367 from phillipp/proper-idle-action-fix

@phillipp thanks for making the changes I requested, I will merge this pull request now.