Log the unexpected string received in place of the SYNC payload length.

src/replication.c

@@ -722,7 +722,7 @@ void readSyncBulkPayload(aeEventLoop *el, int fd, void *privdata, int mask) {
             server.repl_transfer_lastio = server.unixtime;
             return;
         } else if (buf[0] != '$') {
-            redisLog(REDIS_WARNING,"Bad protocol from MASTER, the first byte is not '$', are you sure the host and port are right?");
+            redisLog(REDIS_WARNING,"Bad protocol from MASTER, the first byte is not '$' (we received '%s'), are you sure the host and port are right?", buf);
             goto error;
         }
         server.repl_transfer_size = strtol(buf+1,NULL,10);