Ensure systemd services are running when doing --tags=start

Fixes #129 (Github Issue).

Unfortunately, we rely on `service_facts`, which is only available
in Ansible >= 2.5.

There's little reason to stick to an old version such as Ansible 2.4:
- some time has passed since we've raised version requirements - it's
time to move into the future (a little bit)
- we've recently (in 82b4640072) improved the way one can run
Ansible in a Docker container

From now on, Ansible >= 2.5 is required.

roles/matrix-common-after/tasks/start.yml

@@ -15,4 +15,27 @@
     name: "{{ item }}"
     enabled: yes
     state: started
-  with_items: "{{ matrix_systemd_services_list }}"
+  with_items: "{{ matrix_systemd_services_list }}"
+
+# If we check service state immediately, we may succeed,
+# because it takes some time for the service to attempt to start and actually fail.
+#
+# Waiting too long (30s) may not work for a similar reason,
+# as we may run into systemd's automatic restart logic retrying the service.
+- name: Wait a bit, so that services can start (or fail)
+  wait_for:
+    timeout: 5
+  delegate_to: 127.0.0.1
+  become: false
+
+- name: Populate service facts
+  service_facts:
+
+- name: Fail if service isn't detected to be running
+  fail:
+    msg: >-
+      {{ item }} was not detected to be running.
+      It's possible that there's a configuration problem or another service on your server interferes with it (uses the same ports, etc.).
+      Try running `systemctl status {{ item }}` and `systemctl -fu {{ item }}` on the server to investigate.
+  with_items: "{{ matrix_systemd_services_list }}"
+  when: "ansible_facts.services[item + '.service']|default(none) is none or ansible_facts.services[item + '.service'].state != 'running'"