## 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
constraints = [
            models.UniqueConstraint(fields=["organization_id", "user_id"], name="unique_organization_membership"),
            models.UniqueConstraint(
                fields=["organization_id"], condition=models.Q(level=15), name="only_one_owner_per_organization"
            ),
        ]
    def __str__(self):
        return str(self.Level(self.level))
    def validate_level_change(self, membership_being_updated: "OrganizationMembership", new_level: Level) -> None:
        if membership_being_updated.id == self.id:
            raise exceptions.PermissionDenied("You can't change your own access level.")
        if membership_being_updated.organization_id != self.organization_id:
            raise exceptions.PermissionDenied("You both need to belong to the same organization.")
        if membership_being_updated.level >= self.level:
            raise exceptions.PermissionDenied("You can only change access level of others with level lower than you.")
        if new_level == OrganizationMembership.Level.OWNER:
            if self.level != OrganizationMembership.Level.OWNER:
                raise exceptions.PermissionDenied("You can only pass on organization ownership if you're its owner.")
            self.level = OrganizationMembership.Level.ADMIN
            self.save()
        elif new_level >= self.level:
            raise exceptions.PermissionDenied(
                "You can only change access level of others to lower than your current one."
            )
    __repr__ = sane_repr("organization", "user", "level")
class OrganizationInvite(UUIDModel):