Try to make setup.yml more helpful when galaxy roles haven't been downloaded

With this change, trying to run the playbook should report an error that
looks like this:

> The offending line appears to be:
>
>     # Most of the roles below are not distributed with the playbook, but downloaded separately using `ansible-galaxy` via the `make roles` command (see `Makefile`).
>     - role: galaxy/com.devture.ansible.role.playbook_help
>       ^ here

.. with that comment included, which might help some people who missed
the `make roles` part.

Improves https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2232

setup.yml

@@ -7,10 +7,9 @@
     - roles/custom/matrix-synapse/vars/workers.yml
 
   roles:
-    # This role has no tasks at all
+    # Most of the roles below are not distributed with the playbook, but downloaded separately using `ansible-galaxy` via the `make roles` command (see `Makefile`).
     - role: galaxy/com.devture.ansible.role.playbook_help
 
-    # This role has no tasks at all
     - role: galaxy/com.devture.ansible.role.systemd_docker_base
 
     - role: custom/matrix_playbook_migration