Revert "update v4 metadata to return network stats despite missing stats for single container (#2719)"

This reverts commit 7a82ee7.

Author: fierlion

Diff for: agent/handlers/v4/task_metadata_handler.go

@@ -65,7 +65,12 @@ func TaskMetadataHandler(state dockerstate.TaskEngineState, ecsClient api.ECSCli
 			for _, containerResponse := range taskResponse.Containers {
 				networks, err := GetContainerNetworkMetadata(containerResponse.ID, state)
 				if err != nil {
-					seelog.Warnf("Error retrieving network metadata for container %s - %s", containerResponse.ID, err)
+					errResponseJSON, err := json.Marshal(err.Error())
+					if e := utils.WriteResponseIfMarshalError(w, err); e != nil {
+						return
+					}
+					utils.WriteJSONToResponse(w, http.StatusInternalServerError, errResponseJSON, utils.RequestTypeContainerMetadata)
+					return
 				}
 				containerResponse.Networks = networks
 				responses = append(responses, containerResponse)