Improse FetchError on first byte timeout

Fixes #2185

Test case by @fgsch

bin/varnishd/http1/cache_http1_fetch.c

@@ -199,6 +199,10 @@ V1F_FetchRespHdr(struct busyobj *bo)
 			VSLb(bo->vsl, SLT_FetchError, "overflow");
 			htc->doclose = SC_RX_OVERFLOW;
 			break;
+		case HTC_S_IDLE:
+			VSLb(bo->vsl, SLT_FetchError, "first byte timeout");
+			htc->doclose = SC_RX_TIMEOUT;
+			break;
 		default:
 			VSLb(bo->vsl, SLT_FetchError, "HTC %s (%d)",
 			     HTC_Status(hs), hs);

bin/varnishtest/tests/b00023.vtc

@@ -19,12 +19,18 @@ varnish v1 -vcl+backend {
 	}
 } -start
 
+logexpect l1 -v v1 {
+	expect * 1002   FetchError      "first byte timeout"
+} -start
+
 client c1 {
 	txreq
 	rxresp
 	expect resp.status == 503
 } -run
 
+logexpect l1 -wait
+
 server s1 {
 	rxreq
 	delay 0.2