in roundup/cgi/actions.py
raise exceptions.NotFound(msg) should give the msg to the web browser.
It does not with 4624:21705126dafa (pre 1.4.20).

Should be fixed and then the following code needs to be replaced:
       # check if all columns exist on class
        # the exception must be raised before sending header
        props = klass.getprops()
        for cname in columns:
            if cname not in props:
                # TODO raise exceptions.NotFound(.....) does not give
message
                # so using SeriousError instead
                self.client.response_code = 404
                raise exceptions.SeriousError(
                    self._('Column "%(column)s" not found on
%(class)s')
                    % {'column': cgi.escape(cname), 'class':
request.classname})

in roundup/cgi/actions.py to use NotFound().