There is an error.php in the "system" template folder, which contains "$this->error->getMessage()" three times (title meta tag, title of error box and as a part of the content under "Technical info" ).

I guess I first have to create my own copy of the error.php in my template folder.

EDIT: OK, this works, but the possibilities are very limited, so I now use a completely different way of creating my own error document (via an article, modification of PHP code in error.php and in .htaccess).