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latest version simply returned an exception if the db was out of date.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 random
exceptions (exit code 1) anymore.
Merge pull request #561 from dosomder/557-fix-latest-version

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