Do not allow returns with arguments in generators

[JukkaL]

The following is accepted by the compiler, even though it is not valid in Python:

def f():
    yield 1
    return 1

There should be a compile error if mixing return with an argument and yield within the same function.

Update: only disallow this in Python 2 (see below).

[etrepum]

This is invalid in Python 2 (and the existing Python 2.7.8 compiler will give you a SyntaxError for this), but valid in Python 3 due to PEP 380 http://legacy.python.org/dev/peps/pep-0380/

[JukkaL]

Okay, only disallow this in Python 2 mode?

[etrepum]

Yeah, it's a feature and not a bug in Python 3 :)