Add CORS error message to PHP requests

[bgrgicak]

Let's log a detailed error message when HTTP request made by PHP fail due to a CORS error, e.g. like in this blueprint that calls wp_safe_remote_get('https://wordpress.org');.

Playground Web is limited when making HTTP requests to sites that support CORS. Because Playground runs in a browser it can only send requests to servers that allow CORS requests. A typical PHP server doesn't have this restriction.

When this happens the request fails silently. We should find a way of telling the developer what went wrong, it could even be an mu-plugin that overrides window.fetch with a version that logs an extra error message discussing CORS.

Related issues #1349 #772