How to display the message passed to Http404 in a custom 404 Error template in Django?

I have a project that I am working on in django. There are a lot of instances where I:

raise Http404("this is an error")

and it creates a nice 404 page for me with the error message "this is an error" written on it.

I now want to create a custom error page and have it still display the message, but I can't figure out how.

I'm sure it's just a template variable that I need to add to my custom 404 template, but I can't find any documentation for it.