## 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/models/organization.py
members: models.ManyToManyField = models.ManyToManyField(
        "posthog.User",
        through="posthog.OrganizationMembership",
        related_name="organizations",
        related_query_name="organization",
    )
    name: models.CharField = models.CharField(max_length=64)
    created_at: models.DateTimeField = models.DateTimeField(auto_now_add=True)
    updated_at: models.DateTimeField = models.DateTimeField(auto_now=True)
    objects = OrganizationManager()
    @property
    def _billing_plan_details(self) -> Tuple[Optional[str], Optional[str]]:
        """
        Obtains details on the billing plan for the organization.
        Returns a tuple with (billing_plan_key, billing_realm)
        """
        
        if hasattr(self, "billing"):
            if self.billing is None:  
                return (None, None)
            return (self.billing.get_plan_key(), "cloud")  
        
        if License is not None:
            license = License.objects.filter(valid_until__gte=timezone.now()).first()
            if license:
                return (license.plan, "ee")
        return (None, None)
    @property
    def billing_plan(self) -> Optional[str]: