Fix Postgres import when Postgres had never started

If this is a brand new server and Postgres had never started,
detecting it before we even start it is not possible.

This moves the logic, so that it happens later on, when Postgres
would have had the chance to start and possibly initialize
a new empty database.

Fixes #82 (Github issue)

roles/matrix-postgres/tasks/import_postgres.yml

@@ -22,13 +22,6 @@
     msg: "File cannot be found on the server at {{ server_path_postgres_dump }}"
   when: not result_server_path_postgres_dump_stat.stat.exists
 
-- import_tasks: tasks/util/detect_existing_postgres_version.yml
-
-- name: Abort, if no existing Postgres version detected
-  fail:
-    msg: "Could not find existing Postgres installation"
-  when: "not matrix_postgres_detected_existing"
-
 
 # Defaults
 
@@ -52,6 +45,13 @@
   delegate_to: 127.0.0.1
   become: false
 
+- import_tasks: tasks/util/detect_existing_postgres_version.yml
+
+- name: Abort, if no existing Postgres version detected
+  fail:
+    msg: "Could not find existing Postgres installation"
+  when: "not matrix_postgres_detected_existing"
+
 - name: Perform Postgres database import
   command: |
     /usr/bin/docker run --rm --name matrix-postgres-import \