- BS_EXCP in generate_exception_err and after hen_helper_wait
becomes DISAS_NORETURN, because we do not return after
raising an exception.
- Some uses of BS_EXCP are misleading in that they're used
only as a "not BS_STOP" exit condition, i.e. they have nothing
to do with an actual exception. For those cases, define
and use DISAS_EXIT, which is clearer. With this and the
above change, BS_EXCP goes away completely.