Htaccess tips **EDITED 03/01/2019**

I've edited a .htaccess file before, to redirect missing pages (i.e a 404 code)viathe ErrorDocument redirect commands,but whenever the 500 error occurs, the default message is displayed from the browser and not the page i want to actually display.

Is there a way to ensure that the redirected page actually appears as directed by the file, or does it have to be redirected to a different webserver?