Fix exception-related PEP8 issues

This changeset resolves 3 PEP8 issues: It addresses the following
exception-related PEP8 violation:
- W602 deprecated form of raising exception
Yeah, thanks for requesting a clarification. It's kind of wonky and took
a good bit of searching/reading [1], so it's a good thing to share. I
thought I originally had it correct after I first read through, and my
initial implementation blew the stack. I think I've got it now...

Grammar:
```
raise_stmt ::=  "raise" [expression ["," expression ["," expression]]]
```

We're using the "third" form here (providing 3 parameters).

> Otherwise, raise evaluates the expressions to get three objects, using
> None as the value of omitted expressions. The first two objects are
> used to determine the type and value of the exception.

> If the first object is an instance, the type of the exception is the
> class of the instance, the instance itself is the value, and the
> second object must be None.

> If a third object is present and not None, it must be a traceback
> object (see section The standard type hierarchy), and it is
> substituted instead of the current location as the place where the
> exception occurred.

[1] https://docs.python.org/2/reference/simple_stmts.html#raise