Clarified  error messages of $active_group value (error msgs in DB.php)

Used the load->model wrong (had 3rd param as class alias, not second) and spent 30 minutes tracking down the "You have specified an invalid database connection" error rendered from DB.php.  
I modified the code to have two different error messages, one unset $active_group, and the other to display the active_group it was looking for.