display a proper error page when social provider is loaded but not configured

[luto]

DoesNotExist at /accounts/google/login/ SocialApp matching query does not exist.

File "/opt/dino/.local/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
  34.             response = get_response(request)

File "/opt/dino/.local/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  115.                 response = self.process_exception_by_middleware(e, request)

File "/opt/dino/.local/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  113.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/opt/dino/.local/lib/python3.6/site-packages/allauth/socialaccount/providers/oauth2/views.py" in view
  73.                 return self.dispatch(request, *args, **kwargs)

File "/opt/dino/.local/lib/python3.6/site-packages/allauth/socialaccount/providers/oauth2/views.py" in dispatch
  96.         app = provider.get_app(self.request)

File "/opt/dino/.local/lib/python3.6/site-packages/allauth/socialaccount/providers/base.py" in get_app
  52.         return SocialApp.objects.get_current(self.id, request)

File "/opt/dino/.local/lib/python3.6/site-packages/allauth/socialaccount/models.py" in get_current
  34.                 provider=provider)

File "/opt/dino/.local/lib/python3.6/site-packages/django/db/models/manager.py" in manager_method
  82.                 return getattr(self.get_queryset(), name)(*args, **kwargs)

File "/opt/dino/.local/lib/python3.6/site-packages/django/db/models/query.py" in get
  408.                 self.model._meta.object_name

Exception Type: DoesNotExist at /accounts/google/login/
Exception Value: SocialApp matching query does not exist.