kiji-schema: SCHEMA-802. Log warning when uninstalling instances and deleting tables with registered users

issue: https://jira.kiji.org/browse/SCHEMA-802
review: https://review.kiji.org/r/1898
commit: https://github.com/kijiproject/kiji-schema/commit/9d47418fa83c8e5c5e756f651d13f4ba02a2fdb1