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. 
In the spirit of #1605, errors should just be []error instead of ErrorList to ensure that errors can be introspected but maintain the property that an empty errors won't be construed an error.