Use Language on Error Handling - show_error()

I must admit that it seems to me quite complex especially if I have to create a specific and class page for each of the error pages (error_404, error_db, error_exception, error_general, ...)

I never indeed use these functions extension. I would just like to personalize all these pages simply.
I would just like to be able to load a view ( $this->load->view('header'); ) in "gml_application/views/errors/html/error_404.php" but it also doesn't work.

I guess it's because of CI_Controller not being loaded, I may not know how to load it either. It is all very technical.