Fallback to AliasBasedTokenStateService in case of DB errors

We recently added support to pure JDBC-based token state management. It'd be great to make deployments smooth with the following safety net: if there is (are) any issue(s) with the configured DB connection, the Knox Gateway should start using the AliasBasedTokenStateService instead of fail-fast. Additionally, the tokengen UI should indicate the fact if the underlying token back-end is the one the user configured.