Add a explicit error when fact_caching_connection is not set

By default, jsonfile is not documented, and the error message
when fact_caching_connection is not set is a bit puzzling, so
a error message would be beeter ( documentation too ). While redis
is faster for bigger setup, jsonfile is fine for a small setup
and is easier to deploy.

The module will then stop ansible-playbook, as this match better
the philosophy of Ansible being a fail-fast system.