Backported 1.39 and 1.40 from trunk:

Backported 1.39 and 1.40 from trunk:

1.39:
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.

1.40:
Two more cases of switch(PySequence_Size()) without checking for case -1.
(Same problem as last checkin for SF bug 610610)
Need to clear the error and proceed.