when a slave experiances an error on commands that come from master, print to the log

since slave isn't replying to it's master, these errors go unnoticed.
since we don't expect the master to send garbadge to the slave, this should be safe.
(as long as we don't log OOM errors there)

src/networking.c

@@ -376,6 +376,8 @@ void addReplyErrorLength(client *c, const char *s, size_t len) {
     addReplyString(c,"-ERR ",5);
     addReplyString(c,s,len);
     addReplyString(c,"\r\n",2);
+    if (c->flags & CLIENT_MASTER)
+        serverLog(LL_WARNING,"Error sent to master: %s", s);
 }
 
 void addReplyError(client *c, const char *err) {