While installation I used a private mirror which was misconfigured.
I got 403 when downloading grub or lilo, but the installer did not told
something about misconfigured sources.
Maybe the error message can be improved.
That would mean analyzing the possible cause of failure when trying to
get packages....which is a somewhat complicated task because of the
various possible errors that can happen on private mirrors.
IMHO, having the installer say that it failed to retrieve this or that
package is the only reasonable solution....which is what we're doing
right now.