Make sure Etherpad has a database to write to

roles/matrix-etherpad/tasks/validate_config.yml

@@ -3,3 +3,9 @@
     msg: >-
       To integrate Etherpad notes with Matrix rooms you need to set "matrix_dimension_enabled" to true
   when: "not matrix_dimension_enabled|bool"
+
+- name: Fail if no database is configured for Etherpad
+  fail:
+    msg: >-
+      Etherpad requires a dedicated Postgres database. Please enable the built in one, or configure an external DB by redefining "matrix_etherpad_database_hostname"
+  when: matrix_etherpad_database_hostname == "matrix-postgres" and not matrix_postgres_enabled