Going to this: /bin/wcmcommand?cmd=copyPage on our authoring instance results in a 500 message. However, it is using the default 500 page, rather than the custom pages I have at /apps/sling/servlet/errorhandler. I can confirm they work because going to /bin/wcmcommand goes to those pages - note that it does not have a post request of cmd=copyPage.
How can I get it to use my custom page?