## 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
created_by_id = serializers.IntegerField(source="created_by.id", read_only=True)
    created_by_email = serializers.CharField(source="created_by.email", read_only=True)
    created_by_first_name = serializers.CharField(source="created_by.first_name", read_only=True,)
    
    target_email = serializers.CharField(required=True)
    class Meta:
        model = OrganizationInvite
        fields = [
            "id",
            "target_email",
            "created_by_id",
            "created_by_email",
            "created_by_first_name",
            "created_at",
            "updated_at",
            "emailing_attempt_made",
        ]
        read_only_fields = [
            "id",
            "created_by_id",
            "created_by_email",
            "created_by_first_name",
            "created_at",
            "updated_at",
            "emailing_attempt_made",
        ]
    def create(self, validated_data: Dict[str, Any], *args: Any, **kwargs: Any) -> OrganizationInvite: