simplify util.ErrorList by eliminating errorListInternal

This refactor makes ErrorList introspectable as per Issue 1605 https://github.com/GoogleCloudPlatform/kubernetes/issues/1605. It passes make check. 
Thanks. Yes, I see what you mean. I'll roll-back errorListInternal. There does remain the question of the utility of the public ErrorList (see below). []error and errorListInternal do serve their particular roles. Would you be amenable to removing the public ErrorList altogether?