[#108] object/head: Return 404 error if header was not found

Define ErrNotFound error in headsvc package. Return ErrNotFound from Head
method if the header was not found in the container.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/object/head/distributed.go

@@ -135,7 +135,7 @@ loop:
 	}
 
 	if !h.traverser.Success() {
-		return nil, errors.Errorf("(%T) incomplete object Head operation", h)
+		return nil, errors.Wrapf(ErrNotFound, "(%T) incomplete object Head operation", h)
 	}
 
 	return resp, nil

pkg/services/object/head/service.go

@@ -39,6 +39,8 @@ type cfg struct {
 	rightChildSearcher RelationSearcher
 }
 
+var ErrNotFound = errors.New("object header not found")
+
 func defaultCfg() *cfg {
 	return &cfg{
 		workerPool: new(util.SyncWorkerPool),