The raise logic has one additional feature: if you raise <class>,
<value> where <value> is not an instance, it will construct an
instance using <value> as argument.  If <value> is None, <class> is
instantiated without arguments.  If <value> is a tuple, it is used as
the argument list.

This feature is intended to make it easier to upgrade code from using
string exceptions to using class exceptions; without this feature,
you'd have to change every raise statement from ``raise X'' to ``raise
X()'' and from ``raise X, y'' to ``raise X(y)''.  The latter is still
the recommended form (because it has no ambiguities about the number
of arguments), but this change makes the transition less painful.