beresp.backend.ip is only available while the connection is open,

which means not in vcl_backend_error{}

Fixes: #1865

Conflicts:
	lib/libvcc/generate.py

lib/libvcc/generate.py

@@ -587,7 +587,7 @@ sp_variables = [
 	),
 	('beresp.backend.ip',
 		'IP',
-		( 'backend_response', 'backend_error'),
+		( 'backend_response', ),
 		( ), """
 		IP of the backend this response was fetched from.
 		"""