* "-1" is an invalid nonce error, show 'An error has occurred. Please reload the page and try again.'.
* "error" means that the connection to the server was lost, show 'Connection lost or the server is busy. Please try again later.'.
* Escape the message in `wp-updates-admin-notice` because the response may include HTML.
* Remove HTML tags in `wp.updates.isValidResponse()` to make PHP's error messages more readable.
Props azaozz for review.
Fixes#37583.
git-svn-id: https://develop.svn.wordpress.org/trunk@38205 602fd350-edb4-49c9-b593-d223f7449a82