Feature: raise ValueError if wrapped is None in wsgiapp/wsgiapp2

This is an issue I ran into (mainly because I am not that bright ...) I was failing to `return config.make_wsgi_app()` in my application, I was trying to wrap `None` by calling (`wsgiapp2(None)`), as soon as I attempted to access the view that was registered, which would hang the request, forever (pserve would use up all of it's threads, and then it would serve no more!)

This just adds some extra protection by raising an `ValueError` instead.