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.

Doc/library/exceptions.rst

@@ -288,7 +288,7 @@ The following exceptions are the exceptions that are usually raised.
 
    .. versionchanged:: 3.3
       :exc:`EnvironmentError`, :exc:`IOError`, :exc:`WindowsError`,
-      :exc:`VMSError`, :exc:`socket.error`, :exc:`select.error` and
+      :exc:`socket.error`, :exc:`select.error` and
       :exc:`mmap.error` have been merged into :exc:`OSError`, and the
       constructor may return a subclass.
 

Doc/library/unittest.mock.rst

@@ -1697,7 +1697,7 @@ Methods and their defaults:
 * ``__ge__``: NotImplemented
 * ``__int__``: 1
 * ``__contains__``: False
-* ``__len__``: 1
+* ``__len__``: 0
 * ``__iter__``: iter([])
 * ``__exit__``: False
 * ``__complex__``: 1j