Psych::Visitors::ToRuby#build_exception

static VALUE build_exception(VALUE self, VALUE klass, VALUE mesg)
{
    VALUE e = rb_obj_alloc(klass);

#ifdef TRUFFLERUBY
    rb_exc_set_message(e, mesg);
#else
    rb_iv_set(e, "mesg", mesg);
#endif

    return e;
}