Automatic merge from submit-queue

Report validation errors when command-line flag parsing fails.

Before this, I was stumped with

 invalid argument "federation=kubernetes-federation.test." for --federations=federation=kubernetes-federation.test.: federation not a valid federation name

but now

  invalid argument "federation=kubernetes-federation.test." for --federations=federation=kubernetes-federation.test.: federation not a valid federation name: ["must match the regex [a-z0-9]([-a-z0-9]*[a-z0-9])?(\\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)* (e.g. 'example.com')"]