Support Unwrap for hcserror

Go 1.13 introduced the Unwrap method which allows examining wrapped errors more-cleanly via errors.Is.

It'd be handy to support that on hcsshim.HcsError to support cases like

	// ERROR_DEV_NOT_EXIST is returned if the layer is not currently prepared.
	if err := hcsshim.UnprepareLayer(info, filepath.Base(layerPath)); err != nil {
		if hcserror, ok := err.(*hcsshim.HcsError); !ok || hcserror.Err != windows.ERROR_DEV_NOT_EXIST {
			return errors.Wrapf(err, "failed to unprepare %s", layerPath)
		}
	}

which could just be

	// ERROR_DEV_NOT_EXIST is returned if the layer is not currently prepared.
	if err := hcsshim.UnprepareLayer(info, filepath.Base(layerPath)); err != nil && !err.Is(windows.ERROR_DEV_NOT_EXIST) {
		return errors.Wrapf(err, "failed to unprepare %s", layerPath)
	}

.

(See https://github.com/containerd/containerd/pull/5133)

It should be a fairly easy addition, since HcsError already carries a public Err field.

Even if we still need to support Go 1.12 and earlier, this would provide compatibility with the golang.org/x/xerrors backfill, and won't otherwise hurt.

Start by locating the HcsError definition and checking how its public Err field is currently exposed; then run the package's existing tests. Done means HcsError supports Go error unwrapping so callers can use errors.Is or xerrors compatibility with the underlying error.