Upgrade/Install: Adjust the check for missing extensions during an upgrade so it remains compatible with versions of WordPress prior to 5.1.

The `WP_Error::has_errors()` method was introduced in WordPress 5.1, so this change uses its internal logic instead to remain compatible with earlier versions.

Props swissspidy, joemcgill, desrosj.

Fixes #63052
Built from https://develop.svn.wordpress.org/trunk@59956


git-svn-id: http://core.svn.wordpress.org/trunk@59298 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/update-core.php

@@ -1199,7 +1199,7 @@ function update_core( $from, $to ) {
 		}
 
 		// Add a warning when required PHP extensions are missing.
-		if ( $missing_extensions->has_errors() ) {
+		if ( ! empty( $missing_extensions->errors ) ) {
 			return $missing_extensions;
 		}
 	}