Replaced 'raise NotImplemented' with 'raise NotImplementedError' globally.

Note that 'raise NotImplemented' is not syntactic:

    >>> raise NotImplemented
    Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    TypeError: exceptions must be old-style classes or derived from BaseException, not NotImplementedType

But note that 'raise NotImplementedError' is ok:

    >>> raise NotImplementedError
    Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    NotImplementedError