## 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
class Level(models.IntegerChoices):
        MEMBER = 1, "member"
        ADMIN = 8, "administrator"
        OWNER = 15, "owner"
    organization: models.ForeignKey = models.ForeignKey(
        "posthog.Organization", on_delete=models.CASCADE, related_name="memberships", related_query_name="membership"
    )
    user: models.ForeignKey = models.ForeignKey(
        "posthog.User",
        on_delete=models.CASCADE,
        related_name="organization_memberships",
        related_query_name="organization_membership",
    )
    level: models.PositiveSmallIntegerField = models.PositiveSmallIntegerField(
        default=Level.MEMBER, choices=Level.choices
    )
    joined_at: models.DateTimeField = models.DateTimeField(auto_now_add=True)
    updated_at: models.DateTimeField = models.DateTimeField(auto_now=True)
    class Meta: