Add a better error message for service

While migrating my playbook to a newer ansible version, I faced
the error message "unknown init system, cannot enable service". It turned
out to be caused by a wrong service name that was not expanded anymore.

So by giving the name of the service that cannot be enabled and a more precise
reason, i think people will be able to diagnose their issue more easily.
Merge pull request #7076 from mscherer/better_message_service

Add a better error message for service