[new user ux] Nice message if knative not installed

[drnic]

Currently if a user targets a cluster without knative, or they've forgotten to install knative, we give an obscure message about resources:

$ knctl deploy \
      --namespace helloworld \
      --service hello \
      --image gcr.io/knative-samples/helloworld-go \
      --env TARGET=Rev1

Error: the server could not find the requested resource (get revisions.serving.knative.dev)

Perhaps instead we could indicate they are either targeting the wrong kubernetes cluster or Run "knctl install" to install Knative into your cluster