apt-get update: exit code 0 even when failing

from my "bug-report for apt":bugs.debian.org/cgi-bin/bugreport.cgi?bug=696335

apt-get update still exits with an exit-code 0, even if it fails (i.e. due to network connection error).

so in the meantime we need a little wrapper script that:

• executes apt-get update
• parses the output for possible errors
• returns a adequate exit code

(from redmine: created on 2012-12-19, closed on 2013-04-02, relates #1091, relates #1609)