How to return database errors ?

[eluser]bhenbe[/eluser]
I'm sorry but i don't find any function example wich return a sql error.

I already read the user guide.

I thought i can find an answer in the Managing Errors chapter :

if ($this->db->trans_status() === FALSE)
{
// generate an error (how ?)... or use the log_message() function to log your error (how ?)
}

Perhaps, i need to translate this page to understand...

CI manages sql error without debug mode or not ?

Thank you.