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.
diff --git a/Objects/abstract.c b/Objects/abstract.c
index a8823eb..cac767f 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -912,6 +912,7 @@
       if(err) return -1;
       if(! not_equal) return i;
     }
+  PyErr_SetString(PyExc_ValueError, "list.index(x): x not in list");
   return -1;
 }