http1_deliver: Log client write error

There is otherwise nothing but Debug logs when send_timeout triggers in
the request logs, and only the session logs can help understand partial
deliveries.

This introduces a client counterpart for:

    FetchError backend write error: %d (%s) (%s)

bin/varnishd/http1/cache_http1_deliver.c

@@ -142,8 +142,11 @@ V1D_Deliver(struct req *req, struct boc *boc, int sendbody)
 
 	if (sc == SC_NULL && err && req->sp->fd >= 0)
 		sc = SC_REM_CLOSE;
-	if (sc != SC_NULL)
+	if (sc != SC_NULL) {
+		VSLb(req->vsl, SLT_Error, "client write error: %d (%s) (%s)",
+		    errno, VAS_errtxt(errno), sc->desc);
 		Req_Fail(req, sc);
+	}
 	if (req->doclose == SC_NULL)
 		req->doclose = sc;
 }

bin/varnishtest/tests/r03189.vtc

@@ -14,10 +14,14 @@ server s1 {
 	chunkedlen 0
 } -start
 
-varnish v1				\
-	-arg "-p idle_send_timeout=.1"	\
-	-arg "-p send_timeout=.8"	\
-	-vcl+backend { } -start
+varnish v1 -cliok "param.set idle_send_timeout .1"
+varnish v1 -cliok "param.set send_timeout .8"
+varnish v1 -vcl+backend { } -start
+
+logexpect l1 -v v1 {
+	expect * * Error	"client write error: .* .Error transaction."
+	expect 0 = Timestamp	"Resp:"
+} -start
 
 client c1 {
 	txreq
@@ -26,3 +30,5 @@ client c1 {
 	barrier b sync
 	expect_close
 } -run
+
+logexpect l1 -wait