[#76126570] Check for variations on 'Guru Meditation' errors 

Our current CDN vendor, Fastly, currently use a custom error page in which there is a typo. We'd normally expect a 'Guru Meditation' error, but the typo results in a 'Guru Mediation' error.

Work around this by comparing the HTML response body with an array of expected strings.
Amended in af03f42.