Some consistency in validation returns.

Public functions return a raw slice, internal helpers return errorList.
Yeah, @brendandburns and I had that conversation while you were out, @thockin. I'm not sure if @brendandburns was completely convinced. I won't repeat my arguments here unless requested.

This is a direct link to the relevant thread: https://github.com/GoogleCloudPlatform/kubernetes/pull/691#discussion_r15562880