Use an exception class when raising an exception

[DamianZaremba]

On Python2.7 this kind of works, on Python3.{5,6} it explodes:

python2.7
>>> data = 'something'
>>> raise 'Invalid data!', data
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions must be old-style classes or derived from BaseException, not str

>>> raise ValueError('Invalid data', data)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: ('Invalid data', 'something')

python3.6
>>> data = 'something'
>>> raise 'Invalid data!', data
  File "<stdin>", line 1
    raise 'Invalid data!', data
                         ^
SyntaxError: invalid syntax

>>> raise ValueError('Invalid data', data)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: ('Invalid data', 'something')

I might come back with some more 3.x compatibility items, but this is stopping the code even byte compiling!

Not 100% satisfied with the chosen exception, but it kind of makes sense for the purposes of the raise.

[TimEvens]

Thanks!