The per-tenant results in the tenants[] array are untyped in the

OpenAPI spec (additionalProperties: true). We need to reliably detect

partial failures, where the tenant is created but some of the initial

users are not, since that returns a 200 either way.

Specifically: is the failed-user list new_users_failed_emails or

failed_user_emails? The invite endpoint uses the first and

DistributorCreateTenantOutput uses the second, so we currently check

both to be safe.