Improve error messages from forgetting to call .as_view() on a CBV

[Angus Holder]

We can detect early-on that the user has forgotten to call .as_view() on their CBV when passing it into path(). For:

urlpatterns = [
    path('home', HomeView)
]

The error currently happens only when you first load the route (rather than when constructing the routes), and looks like

Internal Server Error: /
Traceback (most recent call last):
  File "C:\Users\Angus\.virtualenvs\django-WBTbdxDv\lib\site-packages\django\core\handlers\exception.py", line 47, in inner
    response = get_response(request)
  File "C:\Users\Angus\.virtualenvs\django-WBTbdxDv\lib\site-packages\django\core\handlers\base.py", line 179, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
TypeError: __init__() takes 1 positional argument but 2 were given

Which is especially hard to work out given that the traceback doesn't even include any of the user's own code, and this is an error that's easy for beginners to run into when first using CBVs.

My PR changes it to fail early, inside the call to django.urls.path(), with a clear error:

URL route 'foo' should pass in 'EmptyCBView.as_view()' instead of 'EmptyCBView'