Domain Management: add domain mapping DNS error messaging

[designsimply]

Steps to reproduce:

1. Go to https://wordpress.com/domains/add and select any site
2. Add domain mapping for a domain that was purchased elsewhere with the name servers not yet pointed to ns1.wordpress.com, ns2.wordpress.com, or ns3.wordpress.com
3. Complete the purchase
4. Go to https://wordpress.com/domains/manage
5. Click on the domain mapping
6. Look for any kind of indicator noting how to make the mapped domain work

Result: there are no help messages or indicators pointing out that the name servers must be mapped to WordPress.com in order for a mapped domain to work—the domain will not work until the DNS at the registrar where the domain is being managed is updated and has time to propagate, but if you are not familiar with how domains work then it's difficult to figure that out.

Comment from @breezyskies

Suggested copy:

Comment from @umurkontaci

Do we have an endpoint that shows whether the domain is pointing to our NSes?

[designsimply]

Comment from @klimeryk

Yup, there's the /domains/:domain/check-nameservers endpoint 👍

Note (for reference), you can find a list of endpoints by entering /help on the console at https://developer.wordpress.com/docs/api/console/ but you may need to search the code directly for undocumented endpoints.

[klimeryk]

Fixed by #3329.