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-synapse/templates/synapse/systemd/matrix-synapse.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-synapse
-ExecStartPre=-{{ matrix_host_command_docker }} rm matrix-synapse
+ExecStartPre=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} kill matrix-synapse 2>/dev/null'
+ExecStartPre=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} rm matrix-synapse 2>/dev/null'
 {% if matrix_s3_media_store_enabled %}
 # Allow for some time before starting, so that media store can mount.
 # Mounting can happen later too, but if we start writing,
@@ -56,8 +56,8 @@ ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-synapse \
 			{{ matrix_synapse_docker_image }} \
 			-m synapse.app.homeserver -c /data/homeserver.yaml
 
-ExecStop=-{{ matrix_host_command_docker }} kill matrix-synapse
-ExecStop=-{{ matrix_host_command_docker }} rm matrix-synapse
+ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} kill matrix-synapse 2>/dev/null'
+ExecStop=-{{ matrix_host_command_sh }} -c '{{ matrix_host_command_docker }} rm matrix-synapse 2>/dev/null'
 ExecReload={{ matrix_host_command_docker }} exec matrix-synapse kill -HUP 1
 Restart=always
 RestartSec=30