Catch misconfigured REST Auth password provider during installation

roles/matrix-server/tasks/setup/synapse/ext/setup_synapse_ext_rest_auth.yml

@@ -4,6 +4,11 @@
 # Tasks related to setting up matrix-synapse-rest-auth
 #
 
+- name: Fail if REST Auth enabled, but endpoint not configured
+  fail:
+    msg: "You have enabled the REST Auth password provider, but have not configured its endpoint in the `matrix_synapse_ext_password_provider_rest_auth_endpoint` variable. Consult the documentation."
+  when: "matrix_synapse_ext_password_provider_rest_auth_enabled and matrix_synapse_ext_password_provider_rest_auth_endpoint == ''"
+
 - name: Download matrix-synapse-rest-auth
   get_url:
     url: "{{ matrix_synapse_ext_password_provider_rest_auth_download_url }}"