print non-existent key in configmap

pkg/volume/configmap/configmap.go

@@ -266,9 +266,7 @@ func MakePayload(mappings []v1.KeyToPath, configMap *v1.ConfigMap, defaultMode *
 				if optional {
 					continue
 				}
-				err_msg := "references non-existent config key"
-				glog.Errorf(err_msg)
-				return nil, fmt.Errorf(err_msg)
+				return nil, fmt.Errorf("configmap references non-existent config key: %s", ktp.Key)
 			}
 
 			fileProjection.Data = []byte(content)