get mysql errors text

Hello,
Making rest api using CodeIgniter 3.0.3 I want in case of mysql error to return error description in json return block
In case of

PHP Code:

db_debug = true 


I get error message like on my screen : 

PHP Code:

Error Number: 1054
Unknown column 'userna' in 'field list'
SELECT `id`, `userna` FROM `bp_users` ORDER BY `username` ASC LIMIT 4, 2
Filename: models/Muser.php
Line Number: 219 


if

PHP Code:

db_debug = false 


I did not find how to log these errors.
I googled and found  :  

PHP Code:

 $this->db->call_function('error') and $this->db->call_function('errno') 


 where $this - is current control but they show nothig
 
 Also I found :

 Currently it seems we can't get query error message since _error_messageand _error_number are protected methods.

 
I use 'dbdriver' = 'mysqli'

Which is the good way to get errors text in this case?