#6738839 Update connected, but not transferrable domain error message

Changes proposed in this Pull Request

When a domain is already connected (mapped) to a site but is not transferrable, we don’t show the part of the error message indicating that this domain isn’t transferrable (due to the fact that we used to have separate flows for mapping and transfers).

This PR provides more information to help users understand that the domain they are inquiring about is already mapped and isn’t transferrable.

Testing instructions

Either find a site with a connected domain that isn’t transferrable (ex. with a .ca TLD) or hack the is-available endpoint to return something like the following:

{
    domain_name:"example.com",
    status:"mapped_to_same_site_not_transferrable",
    mappable:"mapped_domain",
    supports_privacy:false,
}

Make sure that you see an error message similar to the following: