Yes this assume the network exists.
If network doesn't exists a fog error is raised : 
"Fog::Errors::Error: The resource 'projects/scoreflex-gaming-platform/zones/europe-west1-b/instances/fog-smoke-test-1375685708' was not found"
I don't have explicit error because servers respond a '200 HTTP code' even if network doesn't exist (unlike a request with a wrong zone or a wrong machine type where servers respond a '400 Bad Request')