Always complain if returning a value if annotated to return None

Mypy doesn't complain about this code that returns a value in a function declared to return `None`, since the return value is `Any`:

```
def f() -> None:
    return g()   # oops, but no error

def g():
    return 1
```

I think that mypy could complain about the return statement even if the value has type `Any`. The current behavior is not inconsistent or incorrect, but probably not optimal.
