(py-postprocess-output-buffer): Return t if an exception was found,

(py-postprocess-output-buffer): Return t if an exception was found,
otherwise return nil.

(py-execute-region): When executing the buffer asynchronously in a
subprocess, if an exception occurred, show both the output buffer and
the file containing the exception, leaving point on the source line
containing bottom-most error in the traceback.  If no exception
occurred, jump to the output buffer (no change).