Simplefix

Adds a callback to report mirror failures, to produce output like this:

Downloading Packages:
[MIRROR] mc-4.8.7-3.fc17.i686.rpm: Status code: 404 for http://download.fedoraproject.org/pub/fedora/linux/updates/17/i386/mc-4.8.7-3.fc17.i686.rpm
[FAILED] mc-4.8.7-3.fc17.i686.rpm: No more mirrors to try - All mirrors were already tried without success                             

Mostly, this is just a noise (user generally does not need to know that there were 404 failures, resolved by a mirror failover). But this helps to debug mirror errors, should explain progress percentages being reset, etc, and users expect this behavior.

Also adds the "throttle" option.