apt-get update exit status

In your example apt-get update didn't exit with error, because it considered the problems as warnings, not as fatally bad. If there's a really fatal error, then it would exit with non-zero status.

One way to recognize anomalies is by checking for these patterns in stderr:

• Lines starting with W: are warnings
• Lines starting with E: are errors

You could use something like this to emulate a failure in case the above patterns match, or the exit code of apt-get update itself is non-zero:

if ! { sudo apt-get update 2>&1 || echo E: update failed; } | grep -q '^[WE]:'; then
    echo success
else
    echo failure
fi

Note the ! in the if. It's because the grep exits with success if the pattern was matched, that is if there were errors. When there are no errors the grep itself will fail. So the if condition is to negate the exit code of the grep.