Accessing the 404 error message in a custom page

[Yan Wong]

I want to have a custom styled page for my 404 errors, so that when I do raise(404, 'bad values') it returns a nicely styled page. I have placed the following line in my routes.py

routes_onerror = [('myapp/404', '/myapp/default/custom_404.html')]

but I don't know how to access the error message itself ('bad values' in this case) within the custom page. It doesn't seem to exist in the request object?

[Yan Wong]

Sorry, I meant a page for HTTP 400 errors, not 404 errors, e.g. when I call raise HTTP(404, 'bad values'), but the principle is the same.