fix email notifications, set type to simple instead of oneshot

roles/mediavault/templates/systemd-backup-service.j2

 [Unit]
 Description=backup-{{ item['name'] }}
+{% if mv_mailer_enabled %}
+OnFailure={{ mv_mailer_service_name }}@%n.service
+{% endif %}
 
 [Service]
-Type=oneshot
+Type=simple
 ExecStart={{ mv_script_path }} \
   --rsync-set-flags "{{ item['rsync_flags'] | d(mv_rsync_flags) }}" \
   --strategy "{{ item['retention_strategy'] | d(mv_retention_strategy) }}" \
   {% if item['src_host'] | d() %}{{ item['src_host'] }}:{% endif %}{{ item['src_dir'] | d(mv_src_dir) }} \
   {% if item['dest_host'] | d() %}{{ item['dest_host'] }}:{% endif %}{{ item['dest_dir'] | d(mv_base_dir + '/' + item['name']) }} \
   {{ mv_base_dir }}/{{ item['exclude_list_name'] | d(mv_exclude_list_name) }}.txt
-{% if mv_mailer_enabled %}
-OnFailure={{ mv_mailer_service_name }}@%n.service
-{% endif %}