Update github.com/Microsoft/hcsshim to v0.8.15

Fixes #4915

Release info: https://github.com/microsoft/hcsshim/releases/tag/v0.8.15

Signed-off-by: Kevin Parsons <kevpar@microsoft.com>

vendor/github.com/Microsoft/hcsshim/computestorage/format.go

@@ -2,9 +2,9 @@ package computestorage
 
 import (
 	"context"
-	"fmt"
 
 	"github.com/Microsoft/hcsshim/internal/oc"
+	"github.com/pkg/errors"
 	"go.opencensus.io/trace"
 	"golang.org/x/sys/windows"
 )
@@ -20,7 +20,7 @@ func FormatWritableLayerVhd(ctx context.Context, vhdHandle windows.Handle) (err
 
 	err = hcsFormatWritableLayerVhd(vhdHandle)
 	if err != nil {
-		return fmt.Errorf("failed to format writable layer vhd: %s", err)
+		return errors.Wrap(err, "failed to format writable layer vhd")
 	}
 	return nil
 }