Never fail when stopping systemd service during (SQLite -> Postgres) migration

We need to suppress systemd service-stopping requests in certain rare
cases like https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/771

That issue seems to describe a case, where a migration from mxisd to
ma1sd was happening (DB files had just been moved), and then we were
attemping to stop `matrix-ma1sd.service` so we could import that database into
Postgres. However, there's neither `matrix-mxisd.service`, nor
`matrix-ma1sd.service` after `migrate_mxisd.yml` had just run, so
stopping `matrix-ma1sd.service` was failing.

roles/matrix-postgres/tasks/util/migrate_db_to_postgres.yml

@@ -112,6 +112,7 @@
   service:
     name: "{{ item }}"
     state: stopped
+  failed_when: false
   with_items: "{{ matrix_postgres_db_migration_request.systemd_services_to_stop }}"
 
 - name: Import {{ matrix_postgres_db_migration_request.engine_old }} database from {{ matrix_postgres_db_migration_request.src }} into Postgres