download_repos.sh does wget -nv
if wget gets a 404 (and other http errors, i guess) it nicely says this, and your make output ends like:

(..)
2010-04-04 15:06:06 ERROR 404: Not Found.
make: *** [core-pkgs] Error 8

however if it cannot connect (tcp timeout or whatever), you just get:
make: *** [core-pkgs] Error 4

see:
$ wget -nv http://mirror.twilightlair.net/arch/core/os/i686/core.db.tar.gz
$

$ wget http://mirror.twilightlair.net/arch/core/os/i686/core.db.tar.gz
--2010-04-04 17:11:15-- http://mirror.twilightlair.net/arch/core/os/i686/core.db.tar.gz
Resolving mirror.twilightlair.net... 208.92.233.45
Connecting to mirror.twilightlair.net|208.92.233.45|:80... failed:
Connection refused.
$

I'm not sure what the right approach is.. some ideas:
1) remove the '-nv' and do egrep -v 'Resolving|Connecting'
2) use pacman to get the files for us? (not an option for the $repo.db.tar.gz file i guess)
3) parse the /etc/pacman.d/mirrorlist or modify pacman to return all possible urls on a 'pacman -Sp'
4) "just use a poper mirror", it's not worth it