raise not inside exception block

[John Vandenberg]

`raise` without any arguments inside an exception block can re-raise the exception.

outside an exception block it causes the following on at least 2.7:

TypeError: exceptions must be old-style classes or derived from BaseException, not NoneType

[John Vandenberg (jayvdb)]

Fix PR: https://github.com/pyflakes/pyflakes/pull/57

[Phil Frost (bitglue)]

This is valid Python code:

def foo():
    raise

def bar():
    try:
        raise Exception()
    except:
        foo()

As such, a `raise` outside an `except:` block isn't an error per se: it depends on the call stack from which its called. Since we can't know that until runtime I don't think it's something Pyflakes can check.