This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

We were only reporting failures for when the async task didn't finish.
We also need to report a failure for when the task finished, but
returned a non-zero exit code.

Fixes a regression introduced in d1b5681ba0.

Looks like `.finished` is a property on the main object, not on some
`.async_result` object.

We were previously trying to reach into `.rc`,
but there's no such thing if `async_result.finished == 0`.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2153

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

This makes it easier to use another container image for
rust-synapse-compress-state, which may be storing the binary at another
path.

Fixes:
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/833

Continuation of 1fca917ad1.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/722