Resolve "local backups: design purge system"

Closes #60 (closed)

roles/ynh_backup/templates/ynh_backup.sh.j2

@@ -13,7 +13,7 @@ _fail() {
 
 _create_ynh_backup() {
   echo "Backing up $today YunoHost data now."
-  yunohost backup create {% if ynh_backup.system %}--system{% endif %}{% if ynh_backup.apps %} --apps{% endif %}{% if ynh_backup.directory %} --output-directory {{ ynh_backup.directory }}/backup_"$today" {% endif %}
+  yunohost backup create {% if ynh_backup.system %}--system{% endif %}{% if ynh_backup.apps %} --apps{% endif %}{% if ynh_backup.directory %} --output-directory {{ ynh_backup.directory }}/backup_"$today" {% endif %} || _fail "can't create a backup"
 
 }
 
@@ -25,7 +25,7 @@ _prune_old_backup() {
     done
     _good "Purging of old backups completed."
   else
-    _fail "There is no old backup to be purged."
+    _good "There is no old backup to be purged."
   fi
 }