[Theme Horse Support Team]

Hi starbeam_m,

This feature is not available in our theme. It needs code customization. We recommended not to change the code because if you change the code then while updating to our new version all your customisation code will be lost. So better make child theme and first unhook the functions and the edit the code.

http://codex.wordpress.org/Child_Themes

Here we have replied many times for error page. View example link

The above example is of interface theme. In the same way you need to do from clean retina theme too.
Thank you!