How to use the dgeni.log.error function in dgeni

fragmentsForId.forEach(function(fragment) {
          var doc = fragment.yaml;
          if (!doc.name || !doc.component) {
            log.error('Doc ' + fragment.filePath +
                      ' expects yaml keys "name" and "component"!');
          }

          doc.id = doc.component + '-' + doc.name;
          doc.fileType = typeTransform[fragment.fileType] || fragment.fileType;
          doc.fileName = fragment.fileName;
          doc.contents = fragment.contents;
          doc.extension = doc.fileType.replace(/^\./,'');

          doc.template = templates[doc.fileType] || 'asset.contents.template',
          doc.outputPath = path.join(contentsFolder, _.template(assetOutputPath, doc));

          doc = (transform[doc.fileType] || _.identity)(doc);

          demoData.files.push(doc);
          pages.push(doc);