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.

Since the Varnish mock configuration already mimics Fastly's behaviour,
it makes sense to extend this to the error page. We can then look at
mimicking other CDN vendors using the mocks as necessary in future.

This is also necessary to make the CI tests pass since we're modelling
our Varnish behaviour on Fastly.
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