quality of life improvement: kea-admin db-version fails on empty db

kea-admin db-version prints the following error:

# kea-admin db-version mysql
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1146 (42S02) at line 1: Table 'keatest.schema_version' doesn't exist

when run on an empty DB (without any schema).

Instead, it should catch the fact that schema_version does not exist and should point user to kea-admin db-init command.

This is a quality of life improvement, so it's not terribly important.