Update/Install: Replace "error" and "-1" failure messages with a more meaningful one.

* "-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.
Built from https://develop.svn.wordpress.org/trunk@38205


git-svn-id: http://core.svn.wordpress.org/trunk@38146 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/update.php

@@ -631,7 +631,7 @@ function maintenance_nag() {
 function wp_print_admin_notice_templates() {
 	?>
 	<script id="tmpl-wp-updates-admin-notice" type="text/html">
-		<div <# if ( data.id ) { #>id="{{ data.id }}"<# } #> class="notice {{ data.className }}"><p>{{{ data.message }}}</p></div>
+		<div <# if ( data.id ) { #>id="{{ data.id }}"<# } #> class="notice {{ data.className }}"><p>{{ data.message }}</p></div>
 	</script>
 	<script id="tmpl-wp-bulk-updates-admin-notice" type="text/html">
 		<div id="{{ data.id }}" class="{{ data.className }} notice <# if ( data.errors ) { #>notice-error<# } else { #>notice-success<# } #>">