Login: improve wording for the error "The site at this address is not a WordPress site"

[designsimply]

Steps to reproduce:

1. Go to the "Log in" screen.
2. Tap "entering your site address".
3. Enter a WordPress site address that will return a 403, 404, or 405 error.
4. Observe the error message is confusing because the site actually might be a WordPress site but it's just experiencing a glitch.

Result: if you enter a site that returns a 403, 404, or 405 error, the on screen error will say:

The site at this address is not a WordPress site. For us to connect to it, the site must use WordPress.

Consider updating it to say:

We were unable to access your site, please contact support for assistance.

Would it be possible to also return the error number such as 403, 404, or 405?

^{Tested with WPiOS 13.5.0.0 beta (TestFlight) on iPhone 6S iOS 13.2.}

_{/hat tip @charliescheer for the suggestion (internal reference: p4a5px-2s4-p2)}

[mindgraffiti]

Removing from UL&S. This one belongs in the XML-RPC Errors project.