Make error about unset matrix_ssl_lets_encrypt_support_email more descriptive

Previously, we'd show an error like this:

{"changed": false, "item": null, "msg": "Detected an undefined required variable"}

.. which didn't mention the variable name
(`matrix_ssl_lets_encrypt_support_email`).

roles/matrix-nginx-proxy/tasks/ssl/setup_ssl_lets_encrypt.yml

@@ -30,8 +30,8 @@
   fail:
     msg: "Detected an undefined required variable"
   with_items:
-    - "{{ matrix_ssl_lets_encrypt_support_email }}"
+    - "matrix_ssl_lets_encrypt_support_email"
-  when: "matrix_ssl_retrieval_method == 'lets-encrypt' and item is none"
+  when: "matrix_ssl_retrieval_method == 'lets-encrypt' and vars[item] is none"
 
 - name: Ensure certbot Docker image is pulled
   docker_image: