Detect and handle when onboarded URL is not an organization

We should detect and handle (log better? notify?) when an onboarded URL belongs to a user instead of an organization for all supported code hosting services (ie https://github.com/this-is-a-user/ vs https://github.com/this-is-an-organization).

Ideally we should also detect it upon onboarding when validating the URL at onboarding.italia.it.