Croak when setting a tied @DB::args

Croak when setting a tied @DB::args

In commit f8c105437 I stopped the setting of a tied @DB::args from
crashing, by untying the array.  Untying an array automatically with-
out saying anything seems a little too much like action at a distance.
For now, let’s just croak, as we can always change it later once we
decide what should happen.  At least that’s better than crashing.