Better error handling when Netlify access token incorrect

[Leon Stafford]

current behaviour, when running via WP_CLI:

PHP Fatal error: Uncaught GuzzleHttp\Exception\ClientException: Client error: POST https://api.netlify.com/api/v1/sites/mystaticsite.net/deploys resulted in a 401 Unauthorized response in /usr/html/wp-content/plugins/wp2static-addon-netlify/vendor/guzzlehttp/guzzle/src/Exception/RequestException.php:113

expected behaviour:

presents user with clear error message of what's gone wrong/how to fix