show an error message if the pager list is empty or if the current node isn't in the list

It would be handy for admins to debug their PHP code if the module showed a warning if:
- the pager list is empty
- the current node is trying to be in a pager, but isn't in the pager's list

custom_pager_build_nav seems to return -1 if either of these happens, but nothing's done with that value.