`kp secret create --service-account` creates secret if service account is missing

[fcaroline2020]

When creating a secret with a service account that does not exist, receive an error but secret is still created.

$ kp secret create secretname --dockerhub username --service-account test
Error: serviceaccounts "test" not found
$ kubectl get secrets
somesecret                     tutorial-cosign-key-pair       tutorial-registry-credentials

AC:

• secret creation should fail if the service account is not available