## 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
from typing import Any, Dict
from django.db.models import QuerySet
from rest_framework import exceptions, mixins, serializers, viewsets
from rest_framework.permissions import IsAuthenticated
from rest_framework_extensions.mixins import NestedViewSetMixin
from posthog.email import is_email_available
from posthog.models import OrganizationInvite, OrganizationMembership
from posthog.permissions import OrganizationAdminWritePermissions, OrganizationMemberPermissions
from posthog.tasks.email import send_invite
class OrganizationInviteSerializer(serializers.ModelSerializer):