Fix ambiguous return value for nipap-passwd --latest-version

Before, the function in authlib.py to check the sqlite database for latest version simply returned an exception if the db was out of date.

In this approach, the function will return false and nipap-passwd will exit with the code 2. This way nipapd.config does not catch other exceptions (exit code 1) anymore.

Fixes https://github.com/SpriteLink/NIPAP/issues/557