Fix typos in mock and exceptions docs

The default value of __len__ is 0, not 1:

    >>> from unittest.mock import MagicMock
    >>> mock = MagicMock()
    >>> len(mock)
    0

Reported by Alex on docs@p.o.

Remove the remaining VMSError reference. VMS support is gone.

diff --cc Doc/library/unittest.mock.rst
Simple merge