Resolve "refactor: improve backup usage README"

Closes #45 (closed)

roles/ynh_backup/tasks/backup.yml

@@ -17,6 +17,11 @@
 # along with this program. If not, see <http://www.gnu.org/licenses/>.        #
 #                                                                             #
 #-----------------------------------------------------------------------------#
+- name: Fail if variables are not used correctly
+  ansible.builtin.fail:
+    msg: You need to define variable ynh_backup.apps and / or ynh_backup.system to True
+  when: ynh_backup.apps is false and ynh_backup.system is false
+
 
 - name: Create backup folder if doesn't already exist
   ansible.builtin.file: