## 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
return self._billing_plan_details[0]
    @property
    def available_features(self) -> List[str]:
        plan, realm = self._billing_plan_details
        if not plan:
            return []
        if realm == "ee":
            if plan not in License.PLANS:
                return []
            return License.PLANS[plan]
        return self.billing.available_features  
    def is_feature_available(self, feature: str) -> bool:
        return feature in self.available_features
    def __str__(self):
        return self.name
    __repr__ = sane_repr("name")
class OrganizationMembership(UUIDModel):