Append warning to search_by_sql/search_named.

Currently Teng allows following with no warnings:

```
my $row = $teng->search_by_sql('SELECT * FROM entry WHERE id = ?', [1], 'user');
$row->delete; # user with id = 1 will be deleted.
```

This patch appends warning when the table name is not exists in the sql.

Or more solution with consideration:

 * no warning option?
 * just die in this cases?