## 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/test/test_organization_invites.py
email = "x@x.com"
        response = self.client.post("/api/organizations/@current/invites/", {"target_email": email})
        self.assertEqual(response.status_code, 201)
        self.assertTrue(OrganizationInvite.objects.exists())
        response_data = response.json()
        response_data.pop("id")
        response_data.pop("created_at")
        response_data.pop("updated_at")
        self.assertDictEqual(
            response_data,
            {
                "target_email": email,
                "created_by_email": self.user.email,
                "created_by_id": self.user.id,
                "created_by_first_name": self.user.first_name,
                "emailing_attempt_made": False,
            },
        )
    def test_delete_organization_invite_only_if_admin(self):