container/vfs: wrap decoder errors

This passes line information and handles strconv errors so it reads better.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/mount.go

@@ -149,7 +149,7 @@ func (p *procPaths) remount(target string, flags uintptr) error {
 	return p.mountinfo(func(d *vfs.MountInfoDecoder) error {
 		n, err := d.Unfold(targetKFinal)
 		if err != nil {
-			if errors.Is(err, ESTALE) {
+			if errors.As(err, new(vfs.UnfoldTargetError)) {
 				return msg.WrapErr(err,
 					fmt.Sprintf("mount point %q never appeared in mountinfo", targetKFinal))
 			}