## 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
organization: models.ForeignKey = models.ForeignKey(
        "posthog.Organization", on_delete=models.CASCADE, related_name="invites", related_query_name="invite"
    )
    target_email: models.EmailField = models.EmailField(null=True, db_index=True)
    created_by: models.ForeignKey = models.ForeignKey(
        "posthog.User",
        on_delete=models.SET_NULL,
        related_name="organization_invites",
        related_query_name="organization_invite",
        null=True,
    )
    emailing_attempt_made: models.BooleanField = models.BooleanField(default=False)
    created_at: models.DateTimeField = models.DateTimeField(auto_now_add=True)
    updated_at: models.DateTimeField = models.DateTimeField(auto_now=True)
    def validate(self, *, user: Optional[Any], email: Optional[str] = None) -> None:
        if not email:
            assert user is not None, "Either user or email must be provided!"
            email = user.email
        if self.is_expired():
            raise ValueError("This invite has expired. Please ask your admin for a new one.")
        if email != self.target_email:
            raise ValueError("This invite is intended for another email address.")
        if OrganizationMembership.objects.filter(organization=self.organization, user=user).exists():
            raise ValueError("User already is a member of the organization.")
        if OrganizationMembership.objects.filter(
            organization=self.organization, user__email=self.target_email
        ).exists():
            raise ValueError("A user with this email address already belongs to the organization.")
    def use(self, user: Any, *, prevalidated: bool = False) -> None: