Fix SF bug 610610 (reported by Martijn Pieters, diagnosed by Neal Norwitz).

The switch in Exception__str__ didn't clear the error if
PySequence_Size() raised an exception.  Added a case -1 which clears
the error and falls through to the default case.

Definite backport candidate (this dates all the way to Python 2.0).

Python/exceptions.c

@@ -288,6 +288,9 @@ Exception__str__(PyObject *self, PyObject *args)
 	    out = NULL;
 	break;
     }
+    case -1:
+        PyErr_Clear();
+        /* Fall through */
     default:
         out = PyObject_Str(args);
         break;