error.c:355
static VALUE
exc_to_s(exc)
    VALUE exc;
{
    VALUE mesg = rb_attr_get(exc, rb_intern("mesg"));

    if (NIL_P(mesg)) return rb_class_path(CLASS_OF(exc));
    if (OBJ_TAINTED(exc)) OBJ_TAINT(mesg);
    return mesg;
}
