BUG#21123 (Slave fails to reconnect on update_slave_list):

 Fixing comments and adding error message if slave fails to register
 on master.

sql/slave.cc

@@ -2016,13 +2016,14 @@ pthread_handler_t handle_slave_io(void *arg)
   {
     /*
       Register ourselves with the master.
-      If fails, this is not fatal - we just print the error message and go
-      on with life.
     */
     thd->proc_info = "Registering slave on master";
     if (register_slave_on_master(mysql))
+    {
+      sql_print_error("Slave I/O thread couldn't register on master");
       goto err;
     }
+  }
 
   DBUG_PRINT("info",("Starting reading binary log from master"));
   while (!io_slave_killed(thd,mi))