Don't write normal operational message to stderr
    
    This prevents sending out unnecessary mails when run from the related
    cron job:
    
       condense-mirrorlogs.cron
    -> condense-mirrorlogs.sh > /dev/null
    -> mirrorlist.py
    
    Additionally, report the failing file name in the case of an error.
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>