Bug#12713 "Error in a stored function called from a SELECT doesn't

cause ROLLBACK of statement", part 1. Review fixes.

Do not send OK/EOF packets to the client until we reached the end of 
the current statement.
This is a consolidation, to keep the functionality that is shared by all 
SQL statements in one place in the server.
Currently this functionality includes:
- close_thread_tables()
- log_slow_statement().

After this patch and the subsequent patch for Bug#12713, it shall also include:
- ha_autocommit_or_rollback()
- net_end_statement()
- query_cache_end_of_result().

In future it may also include:
- mysql_reset_thd_for_next_command().

sql/sql_servers.cc

@@ -239,7 +239,7 @@ bool servers_reload(THD *thd)
   if (simple_open_n_lock_tables(thd, tables))
   {
     sql_print_error("Can't open and lock privilege tables: %s",
-		    thd->net.last_error);
+		    thd->main_da.message());
     goto end;
   }