#631 exports errSkipDesc to ErrSkipDesc so that copy can be skipped for mounting. However, ErrSkipDesc is not a real error that should be returned by any function.

I have looked into the golang built-in package io/fs and found SkipAll and SkipDir. To have consistent naming as the build-in packages, I propose that we should rename ErrSkipDesc to SkipDesc SkipNode (as discussed in comment section).

var SkipAll = errors.New("skip everything and stop the walk")
var SkipDir = errors.New("skip this directory")

@Wwwsylvia @ktarplee Any comments?