[#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.
I think it's useful to retain the `%q` so that the list of expected strings doesn't bleed into the error message. We could also retain the same format of `, got blah` then. For example: http://play.golang.org/p/sIqphd1ckh