Reports from Cloudways indicate that there is an intermittent issue with the WP_Upgrader_Skin class.

PHP Fatal error:  Uncaught Error: Class 'WP_Upgrader_Skin' not found in phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/UpgraderSkin.php:13

I have yet not been able to replicate the issue, but this PR already adds hardening around that code section which potentially avoids fatal errors.