Improve/Fix REST Response on Multisite when an endpoint for a non-existent site is hit.

[nerrad]

Currently if you hit the endpoint for a domain on a site that doesn't exist it will behave the same as on a normal request.

• visit ​https://nonexistentsubdomain.eventsmart.com/wp-json in a REST client.
• Take note of the "Location" header in the response: ​https://eventsmart.com/wp-signup.php?new=nonexistentsite. It's also returned with a HTTP 302

Granted this is kind of edgecase, but I think it'd be better to handle things a bit better on REST requests. The question is how? I think from the standpoint of the REST client it'd be better to just return a 404 and a "site doesnt' exist" message (or something to that affect).

[jnylen0]

I am +1 on fixing this - the REST API should return a JSON 404 in the same style as the rest of our WP_Error responses.

The question is how. May be tricky depending on how early the current 404 is returned.

[TimothyBlynJacobs]

It seems like we might be able to utilize wp_is_json_request and the wp_die improvements to make this simpler to handle.

I think this occurs in ms_load_current_site_and_network()?