Use github.com/pkg/errors

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/util/deep_copy.go

@@ -18,24 +18,25 @@ package util
 
 import (
 	"encoding/json"
-	"fmt"
+
+	"github.com/pkg/errors"
 )
 
 // DeepCopy makes a deep copy from src into dst.
 func DeepCopy(dst interface{}, src interface{}) error {
 	if dst == nil {
-		return fmt.Errorf("dst cannot be nil")
+		return errors.New("dst cannot be nil")
 	}
 	if src == nil {
-		return fmt.Errorf("src cannot be nil")
+		return errors.New("src cannot be nil")
 	}
 	bytes, err := json.Marshal(src)
 	if err != nil {
-		return fmt.Errorf("unable to marshal src: %s", err)
+		return errors.Wrap(err, "unable to marshal src")
 	}
 	err = json.Unmarshal(bytes, dst)
 	if err != nil {
-		return fmt.Errorf("unable to unmarshal into dst: %s", err)
+		return errors.Wrap(err, "unable to unmarshal into dst")
 	}
 	return nil
 }