Remove more hardcoded matrix-postgres references

roles/custom/matrix-dimension/tasks/validate_config.yml

@@ -1,11 +1,13 @@
 ---
+
 - name: Fail if required Dimension settings not defined
   ansible.builtin.fail:
-    msg: >-
-      You need to define a required configuration setting (`{{ item }}`) for using Dimension.
+    msg: >
+      You need to define a required configuration setting (`{{ item.name }}`).
+  when: "item.when | bool and vars[item.name] == ''"
   with_items:
-    - "matrix_dimension_access_token"
-  when: "matrix_dimension_enabled and vars[item] == ''"
+    - {'name': 'matrix_dimension_access_token', when: true}
+    - {'name': 'matrix_dimension_database_hostname', when: "{{ matrix_dimension_database_engine == 'postgres' }}"}
 
 - name: (Deprecation) Catch and report renamed Dimension variables
   ansible.builtin.fail: