PyNUT: Fix error when raising without an Exception.

Raising without a valid exception is invalid:

    >>> raise
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: exceptions must be old-style classes or derived from BaseException, not NoneType
    >>> raise Exception
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    Exception

Changing this to "raise Exception" fixes this problem.