Improve warning messages when a remote connection cannot be established

[jakemgold]

Setting up remote connections is causing some confusion with early testers; I think our warning messages might be a bit too vague.

Can we be a bit more specific in catching and explaining common failure use cases? Like the REST API not being available, not finding a secure connection (even linking to a recommended auth plugin), Distributor not being detected on the other end... ?

[helen]

I'm gonna connect this with #368, moving to 1.6.

[peterwilsoncc]

To get some recent notes on testing external connections:

Automatic setup

These seem pretty good. Somewhat geek oriented with the curl error IDs but I don't this that's entirely a bad thing.

• DNS error
  Unable to connect to site.
  cURL error 28: Connection timed out after 5001 milliseconds (http_request_failed)
• Not WordPress
  Unable to connect to site. The external site is private or not a WordPress site. (rest_api_uri_not_found)
• Distributor not installed
  Distributor not installed on remote site.
• REST API Requires Authentication -- error message is inaccurate
  Distributor not installed on remote site.
• HTTPS certificate error (self signed in testing)
  Unable to connect to site.
  cURL error 60: SSL certificate problem: self-signed certificate (http_request_failed)

Manual setup

These could still do with some work but I suspect they're not seen very frequently as manual set up is behind an additional link.

• DNS error
  No connection found.
• Not WordPress
  No connection found.
• Distributor not installed
  Limited connection established. Distributor not installed on remote site.
  -Push distribution unavailable.
  -Pull distribution limited to basic content, i.e. title and content body.
• REST API Requires Authentication -- no auth
  As above, limited connection
• HTTPS certificate error (self signed in testing)
  No connection found.