In regards to the Theme Editor, within wp-admin.
When I upload a 404.php file, it updates the list of files on the right column, and then displays the new 404.php as '404 Template'.
This is not the case with the other error files I upload to my theme section. My 403.php file does not display '403 Template' in the file list, it just displays the filename..
Although this is more of an annoyance than a request, it would be a nice(easy?) fix to have all the error codes display with the correct text. (eg. 403 Template, 500 Template, etc.)