complain loudly about vtwrite failure

src/cmd/vbackup/vbackup.c

diff --git a/src/cmd/vbackup/vbackup.c b/src/cmd/vbackup/vbackup.c
index 42e4f7b..8e7246c 100644
--- a/src/cmd/vbackup/vbackup.c
+++ b/src/cmd/vbackup/vbackup.c
@@ -450,8 +450,12 @@
 {
 	WriteReq wr;
 
-	if(nwritethread == 0)
-		return vtwrite(z, score, type, buf, n);
+	if(nwritethread == 0){
+		n = vtwrite(z, score, type, buf, n);
+		if(n < 0)
+			sysfatal("vtwrite: %r");
+		return n;
+	}
 
 	wr.p = packetalloc();
 	packetappend(wr.p, buf, n);