## 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
from posthog.models.organization import OrganizationInvite, OrganizationMembership
from .base import APIBaseTest
class TestOrganizationInvitesAPI(APIBaseTest):
    def test_add_organization_invite_email_required(self):
        response = self.client.post("/api/organizations/@current/invites/")
        self.assertEqual(response.status_code, 400)
        response_data = response.json()
        self.assertDictEqual(
            response_data,
            {
                "type": "validation_error",
                "code": "required",
                "detail": "This field is required.",
                "attr": "target_email",
            },
        )
    def test_add_organization_invite_with_email(self):