Fix the errors found by go vet

Signed-off-by: rick <rick@jenkins-zh.cn>

pkg/models/registries/manifest.go

@@ -55,7 +55,7 @@ func (r *Registry) ImageManifest(image Image, token string) (*ImageManifest, err
 			log.Error(statusUnauthorized)
 			return nil, restful.NewError(resp.StatusCode, statusUnauthorized)
 		}
-		log.Error("got response: " + string(resp.StatusCode) + string(respBody))
+		log.Errorf("got response: statusCode is '%d', body is '%s'\n", resp.StatusCode, respBody)
 		return nil, restful.NewError(resp.StatusCode, "got image manifest failed")
 	}