Emit minimal 500 response if vcl_synth{} fails.

With the synth->filters koncept out of the picture this becomes
possible.

Fixes: #3441

bin/varnishd/cache/cache_req_fsm.c

@@ -329,7 +329,9 @@ cnt_synth(struct worker *wrk, struct req *req)
 
 	if (wrk->handling == VCL_RET_FAIL) {
 		VSB_destroy(&synth_body);
-		req->doclose = SC_VCL_FAILURE;
+		(void)VRB_Ignore(req);
+		(void)req->transport->minimal_response(req, 500);
+		req->doclose = SC_VCL_FAILURE; // XXX: Not necessary any more ?
 		VSLb_ts_req(req, "Resp", W_TIM_real(wrk));
 		http_Teardown(req->resp);
 		return (REQ_FSM_DONE);

bin/varnishtest/tests/r01576.vtc

@@ -47,6 +47,8 @@ logexpect l1 -v v1 {
 # This should fail with default params and JIT/no-JIT
 client c1 {
 	txreq -url "/${string,repeat,8192,AB}"
+	rxresp
+	expect resp.status == 500
 	expect_close
 } -run
 

bin/varnishtest/tests/r02339.vtc

@@ -154,6 +154,8 @@ client c1 {
 
 client c1 {
 	txreq -url synth
+	rxresp
+	expect resp.status == 500
 	expect_close
 } -run
 

bin/varnishtest/tests/r02488.vtc

@@ -35,6 +35,8 @@ client c1 {
 	expect resp.http.bar == "bar"
 
 	txreq -url "/empty"
+	rxresp
+	expect resp.status == 500
 	expect_close
 } -run
 

bin/varnishtest/tests/v00051.vtc

@@ -150,6 +150,8 @@ logexpect l1007 -v v1 -g raw {
 
 client c1 {
 	txreq -hdr "foo: synth"
+	rxresp
+	expect resp.status == 500
 	expect_close
 } -run