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.