code: function(str) {
              var output;
              try {
                  output = opts.template(str);
              } catch (e) {
                  e.message = error.badTemplate + ': ' + e.message;
                  return this.emit('error', new PluginError('gulp-concat-filenames', e));
              }
  
              if (typeof output !== 'string') {
                  return this.emit('error', new PluginError('gulp-concat-filenames', error.badTemplate));
              }
              return output;
          }
docs: Make sure template errors reach the output