ExcInfo is a triple of exc_type, exc_value, exc_traceback -
analogous to Python's sys.exc_info().  Previously, we were just
throwing exc_value.

I still don't understand all the rules for when type(exc_value)
is not necessarily exc_type.  But this also makes it easier to
pass exc_traceback around, and should make it possible to make
our handling more consistent.

This commit just changes the runtime; the generated code currently
still expects a Box* to be thrown and will crash.