PySequence_Index(): set exception when object is not found in

sequence, otherwise

operator.indexOf([4, 3, 2, 1], 9) would raise a SystemError!

Note: it might be wise to double check all these functions.  I haven't
done that yet.

Objects/abstract.c

@@ -912,6 +912,7 @@ PySequence_Index(s, o)
       if(err) return -1;
       if(! not_equal) return i;
     }
+  PyErr_SetString(PyExc_ValueError, "list.index(x): x not in list");
   return -1;
 }