[#1710] object: Implement Unwrap() for errIncompletePut

* When sign service calls `SignResponse`, it tries to set v2 status
  to response by unwrapping an error to the possible depth. This wasn't
  applicable for `errIncompletePut` so far as it didn't implement
  `Unwrap()`. Thus, it wasn't able to find a correct status set in error.

Change-Id: I280c1806a008176854c55f13bf8688e5736ef941
Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

pkg/services/object/common/writer/distributed.go

@@ -95,6 +95,10 @@ func (x errIncompletePut) Error() string {
 	return commonMsg
 }
 
+func (x errIncompletePut) Unwrap() error {
+	return x.singleErr
+}
+
 // WriteObject implements the transformer.ObjectWriter interface.
 func (t *distributedWriter) WriteObject(ctx context.Context, obj *objectSDK.Object) error {
 	t.obj = obj