Prevent workers failing if their config doesn't exist

- cherry-pick "Ensure worker config exists in systemd service (#7528)"
  from synapse d74cdc1a42e8b487d74c214b1d0ca575429d546a:
  "check that the worker config file exists instead of silently failing."

roles/matrix-synapse/templates/synapse/systemd/matrix-synapse-worker@.service.j2

@@ -2,6 +2,7 @@
 # c.f. https://github.com/matrix-org/synapse/pull/4662
 [Unit]
 Description=Synapse Matrix Worker
+AssertPathExists={{matrix_synapse_config_dir_path }}/worker.%i.yaml
 After=matrix-synapse.service
 BindsTo=matrix-synapse.service