Add 'Guru Mediation' error page to mock config

Add a 'Guru Mediation' (sic) error page to the Varnish mock
configuration to match Fastly's error page which contains this same
typo.
Use a simple string for empty synthetic response

`{"` is used for mainly for multi-line strings, whereas a simple string
would suffice here:
https://www.varnish-cache.org/docs/3.0/reference/vcl.html