D7511: exchange: eliminate some bytes.format() calls

[Yuya Nishihara]

>      if invalid_includes:
>          raise error.Abort(
> -            _(b"The following includes are not accessible for {}: {}").format(
> -                username, invalid_includes
> -            )
> +            _(b"The following includes are not accessible for %s: %s")
> +            % (username, invalid_includes)

Perhaps, `invalid_includes` is a list, and `b'%s' % a_list` is invalid on py3.