def append_error_to_df(test_result, mse, mae):
    """
    Inserts the error values into the first two rows of the dataframe of the predictions of system for easy visualization
    and for further computations.
    
    Parameters
    ----------
    
    test_result   : utility matrix for the result of the recommender systems on the test set
    
    mse           : mse computed from function evaluate
    
    mae           : mae computed from function evaluate
    
    Returns
    -------
    
    test_result   : modified utility matrix with errors
    """
    
    test_result.insert(0, 'mse_u', mse)
    test_result.insert(0, 'mae_u', mae)
    
    return test_result