I am getting this error while my update never completed fully.
W:Failed to fetch http://ppa.launchpad.net/iaz/battery-status/ubuntu/dists/trusty/main/binary-i386/Packages 404 Not Found
, E:Some index files failed to download. They have been ignored, or old ones used instead.
The error is occuring because the mentioned PPA may not be available now. So, you need to remove it from the sources.list so that apt will ignore it.
From a terminal open the file (using sudo) /etc/apt/sources.list and find the line containing the mentioned PPA. Just put a # in front of the line to comment out the line and the save the file.
Now run sudo apt-get update and everything will be Ok.