Display the correct error if varnish.pc is missing
Display the correct error if varnish.pc is missing

Discussed with mithrandir@.

feld/Varnish-Cache,varnish/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,varnish/Varnish-Cache