Eliminate error and restart as possible returns from vcl_backend_response{}

We may want a better way to return synthetic objects from
vcl_backend_reponse{}, it looks a bit clumsy in the vtc's so far.

bin/varnishd/cache/cache_req_fsm.c

@@ -417,7 +417,7 @@ cnt_fetch(struct worker *wrk, struct req *req)
 		AZ(bo->do_esi);
 		AZ(bo->do_pass);
 
-		VCL_backend_response_method(bo->vcl, wrk, req, bo,
+		VCL_backend_response_method(bo->vcl, wrk, NULL, bo,
 		    bo->beresp->ws);
 
 		if (bo->do_pass)

lib/libvcl/generate.py

@@ -85,7 +85,7 @@ returns =(
 	('miss',		"C", ('error', 'restart', 'pass', 'fetch',)),
 	('lookup',		"C", ('error', 'restart', 'pass', 'deliver',)),
 	('backend_fetch',	"B", ('error', 'fetch', 'pass',)),
-	('backend_response',	"B", ('error', 'restart', 'deliver',)),
+	('backend_response',	"B", ('deliver',)),
 	('deliver',		"C", ('restart', 'deliver',)),
 	('error',		"C", ('restart', 'deliver',)),
 	('init',		"", ('ok',)),