force vtDial to name the offending venti server.
if there isn't one, make that clearer too.

Notes:
Wed Jan 25 07:16:59 EST 2006 rsc
    Don't spend too much effort on libventi.
    Its days are numbered.
    


Reference: /n/sources/patch/applied/vtdial-better-err-msg
Date: Wed Jan 25 11:01:08 CET 2006
Reviewed-by: rsc

--- /sys/src/libventi/client.c	Wed Jan 25 11:00:32 2006
+++ /sys/src/libventi/client.c	Wed Jan 25 11:00:30 2006
@@ -29,12 +29,19 @@
 	if(host == nil)
 		host = "$venti";
 
-	na = netmkaddr(host, 0, "venti");
-	fd = dial(na, 0, 0, 0);
+	if (host == nil) {
+		if (!canfail)
+			werrstr("no venti host set");
+		na = "";
+		fd = -1;
+	} else {
+		na = netmkaddr(host, 0, "venti");
+		fd = dial(na, 0, 0, 0);
+	}
 	if(fd < 0){
 		rerrstr(e, sizeof e);
 		if(!canfail){
-			vtSetError("%s", e);
+			vtSetError("venti dialstring %s: %s", na, e);
 			return nil;
 		}
 	}