message: clean up correctly if there's a failure during updater.schema()
  we need to store the newly created updater in this.updaters before trying to work with it, because if it fails we need to be sure it will get cleaned up.
diff: @@ -29,11 +29,10 @@ class BranchUpdate {
  if (!indexer) {
  return [];
  }
  - let updater = await indexer.beginUpdate(this.branch);
  + let updater = this.updaters[dataSource.id] = await indexer.beginUpdate(this.branch);
  owningDataSource.set(updater, dataSource);
  let newModels = await updater.schema();
  this.schemaModels.push(newModels);
  - this.updaters[dataSource.id] = updater;
  return newModels;
  }
  