## Is your feature request related to a problem?

Right now, if you try to invite someone who's already invited (or a user) you get the following error message:
![image](https://user-images.githubusercontent.com/1727427/98357556-adff4a80-2025-11eb-9f4d-ba748155d036.png)

## Describe the solution you'd like

The server itself does return a useful error message, so we could just display that

## Describe alternatives you've considered



## Additional context



#### *Thank you* for your feature request – we love each and every one!

posthog/api/organization_invite.py
return self.filter_queryset_by_parents_lookups(super().get_queryset()).order_by(self.ordering)
    def filter_queryset_by_parents_lookups(self, queryset) -> QuerySet:
        parents_query_dict = self.get_parents_query_dict()
        self.get_serializer_context()
        if parents_query_dict:
            if parents_query_dict["organization_id"] == "@current":
                parents_query_dict["organization_id"] = self.request.user.organization.id
            try:
                return queryset.filter(**parents_query_dict).select_related("created_by")
            except ValueError:
                raise exceptions.NotFound()
        else:
            return queryset
    def get_serializer_context(self):
        """
        Extra context provided to the serializer class.
        """
        parents_query_dict = self.get_parents_query_dict()
        return {
            **super().get_serializer_context(),
            "organization_id": (
                self.request.user.organization.id
                if parents_query_dict["organization_id"] == "@current"
                else parents_query_dict["organization_id"]
            ),
        }