fdconv(): Do not second guess the error condition returned by

fdconv():  Do not second guess the error condition returned by
    PyObject_AsFileDescriptor() -- it does the same thing everywhere, so
    use it the same way everyone else does so that exceptions are
    consistent.  This means we have less code here, and we do not need to
    resort to hackish ways of getting the Python-visible function name to
    fdconv().