Relocate fail check to a better place

roles/matrix-server/tasks/setup/setup_synapse_pre.yml

@@ -1,5 +1,10 @@
 ---
 
+- name: Fail if Macaroon key is missing
+  fail:
+    msg: "You need to set a secret in the matrix_synapse_macaroon_secret_key variable"
+  when: "matrix_synapse_macaroon_secret_key == ''"
+
 - name: Ensure Matrix Synapse paths exist
   file:
     path: "{{ item }}"