code: function done( error, results ) {
  		if ( error ) {
  			return clbk( error );
  		}
  		if ( results ) {
  			return clbk( null, results );
  		}
  		clbk();
  	}
docs: end FUNCTION onFile() 
  FUNCTION: done( error[, results] )
  Callback invoked upon converting an input file.
  
  @private
  @param {Error|Null} error - error object
  @param {String} [results] - conversion results
  @returns {Void}