Do not report docker kill/rm attempts as errors

These are just defensive cleanup tasks that we run.
In the good case, there's nothing to kill or remove, so they trigger an
error like this:

> Error response from daemon: Cannot kill container: something: No such container: something

and:

> Error: No such container: something

People often ask us if this is a problem, so instead of always having to
answer with "no, this is to be expected", we'd rather eliminate it now
and make logs cleaner.

In the event that:
- a container is really stuck and needs cleanup using kill/rm
- and cleanup fails, and we fail to report it because of error
suppression (`2>/dev/null`)

.. we'd still get an error when launching ("container name already in use .."),
so it shouldn't be too hard to investigate.

roles/matrix-bridge-appservice-webhooks/templates/systemd/matrix-appservice-webhooks.service.j2

@@ -13,8 +13,8 @@ DefaultDependencies=no
 [Service]
 Type=simple
 Environment="HOME={{ matrix_systemd_unit_home_path }}"
-ExecStartPre=-{{ matrix_host_command_docker }} kill matrix-appservice-webhooks
-ExecStartPre=-{{ matrix_host_command_docker }} rm matrix-appservice-webhooks
+ExecStartPre=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} kill matrix-appservice-webhooks 2>/dev/null'
+ExecStartPre=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} rm matrix-appservice-webhooks 2>/dev/null'
 
 # Intentional delay, so that the homeserver (we likely depend on) can manage to start.
 ExecStartPre={{ matrix_host_command_sleep }} 5
@@ -35,8 +35,8 @@ ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-appservice-web
 			{{ matrix_appservice_webhooks_docker_image }} \
 			node index.js -p {{ matrix_appservice_webhooks_matrix_port }} -c /config/config.yaml -f /config/webhooks-registration.yaml
 
-ExecStop=-{{ matrix_host_command_docker }} kill matrix-appservice-webhooks
-ExecStop=-{{ matrix_host_command_docker }} rm matrix-appservice-webhooks
+ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} kill matrix-appservice-webhooks 2>/dev/null'
+ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} rm matrix-appservice-webhooks 2>/dev/null'
 Restart=always
 RestartSec=30
 SyslogIdentifier=matrix-appservice-webhooks