Add warnings about using Borg backup with external Postgres

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1982

roles/matrix-backup-borg/tasks/setup_install.yml

@@ -1,6 +1,13 @@
 ---
 
 - block:
+  - name: Fail with matrix_backup_borg_version advice if Postgres not enabled
+    ansible.builtin.fail:
+      msg: >-
+        You are not running a built-in Postgres server (`matrix_postgres_enabled: false`), so auto-detecting its version and setting `matrix_backup_borg_version` automatically based on that cannot happen.
+        Consider setting `matrix_backup_borg_version` to your Postgres version manually.
+    when: not matrix_postgres_enabled
+
   - ansible.builtin.import_role:
       name: matrix-postgres
       tasks_from: detect_existing_postgres_version

roles/matrix-postgres/tasks/detect_existing_postgres_version.yml

@@ -6,6 +6,12 @@
 # This utility is intentionally not in `tasks/util`, because if it were, it wouldn't be possible
 # to include it in other roles via the import_role module: https://docs.ansible.com/ansible/latest/collections/ansible/builtin/import_role_module.html
 
+
+- name: Fail detection if expectation fails (Postgres not enabled)
+  ansible.builtin.fail:
+    msg: "Trying to detect the version of the built-in Postgres server, but Postgres installation is not enabled (`matrix_postgres_enabled: false`)"
+  when: not matrix_postgres_enabled
+
 - name: Initialize Postgres version determination variables (default to empty)
   ansible.builtin.set_fact:
     matrix_postgres_detection_pg_version_path: "{{ matrix_postgres_data_path }}/PG_VERSION"