don't fatal on errors

[mitchdraft]

experiments that specify invalid namespaces cause glooshot to fail:

{"level":"fatal","ts":"2019-05-13T22:59:09.513Z","logger":"glooshot","caller":"glooshot/main.go:21","msg":"error in setup: for experiment: default.abort-ratings, fault index: 0: supergloo-system.istio does not exist: meshes.supergloo.solo.io \"istio\" not found","version":"helmtest5","stacktrace":"main.main\n\t/Users/mitch/go/src/github.com/solo-io/glooshot/cmd/glooshot/main.go:21\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:200"}