Nested NamespaceVersioning

[nazarewk]

Description

This both fixes the misleading error message for NamespaceVersioning and adds proper (at least according to me) nested namespaces handling.

#4009

[toxinu]

👍

[alanjds]

This breaks my URLs scheme that have another namespace before the versions one, as seem:

# urls.py (root)
urlpatterns = [
    url(r'^api/', include('api.urls', namespace='api')),
]

# api/urls.py (app)
api_v1_patterns = [
    url(r'^', include(router.urls)),
    url(r'^api-auth/', include('rest_framework.urls', namespace='rest_framework'))
]

urlpatterns = [
    url(r'^v1/', include(api_v1_patterns, namespace='v1')),
]

Actual master works. With this PR applied it breaks.

[alanjds]

...maybe in conjunction with #3816 it works, but I had not tested.

[nazarewk]

@alanjds it wouldn't work with master if you had any extra namespace after the version, which imho is they way they should be used

I guess I'll have to come up with some idea to handle both before and after cases.