Return weird behaviour of returning json errors

We need this, so client can get error from stream and not from status
code, which is already 200, because write to ResponseWriter was occured.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

api/server/server.go

@@ -861,11 +861,12 @@ func (s *Server) postImagesPush(eng *engine.Engine, version version.Version, w h
 	useJSON := version.GreaterThan("1.0")
 	name := vars["name"]
 
+	output := utils.NewWriteFlusher(w)
 	imagePushConfig := &graph.ImagePushConfig{
 		MetaHeaders: metaHeaders,
 		AuthConfig:  authConfig,
 		Tag:         r.Form.Get("tag"),
-		OutStream:   utils.NewWriteFlusher(w),
+		OutStream:   output,
 		Json:        useJSON,
 	}
 	if useJSON {
@@ -873,8 +874,11 @@ func (s *Server) postImagesPush(eng *engine.Engine, version version.Version, w h
 	}
 
 	if err := s.daemon.Repositories().Push(name, imagePushConfig); err != nil {
+		if !output.Flushed() {
+			return err
+		}
 		sf := streamformatter.NewStreamFormatter(useJSON)
-		return fmt.Errorf(string(sf.FormatError(err)))
+		output.Write(sf.FormatError(err))
 	}
 	return nil