## 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
NestedViewSetMixin,
    mixins.DestroyModelMixin,
    mixins.CreateModelMixin,
    mixins.ListModelMixin,
    viewsets.GenericViewSet,
):
    serializer_class = OrganizationInviteSerializer
    permission_classes = [IsAuthenticated, OrganizationMemberPermissions, OrganizationAdminWritePermissions]
    queryset = OrganizationInvite.objects.all()
    lookup_field = "id"
    ordering = "-created_at"
    def get_queryset(self):