Improve error message when Knative serving is not installed

[rhuss]

Currently, when the Knative CRDs a missing the error message looks like

./kn service create --image k8spatterns/random-generator:2.0 random
the server could not find the requested resource (post services.serving.knative.dev)

Although the error message is not too bad, I think we can improve it here to be less technical (i.e. removing that post ...) and give some more guidance, like in

Error: Knative serving is not installed on the connected backend. Please verify your Knative installation.

A hint to a potential kn doctor (when implemented via #1) could be added here as well.