How to configure Codeigniter to report all errors?

It depends on your version of CI. For < 2.x edit the top level index.php and adjust the error_reporting function to use E_ALL"

error_reporting(E_ALL);

For >= 2.x edit the top level index.php and make sure ENVIRONMENT is set as:

define('ENVIRONMENT', 'development');

which sets the error_reporting to E_ALL.

Create a .htaccess file in your webroot with the following content

php_flag display_startup_errors on
php_flag display_errors on
php_flag html_errors on
php_flag log_errors on
php_flag ignore_repeated_errors off
php_flag ignore_repeated_source off
php_flag report_memleaks on
php_flag track_errors on
php_value error_reporting -1
php_value log_errors_max_len 0

Hopefully that should enable your logs.
Make sure to set the values to off and reporting to 0 on your production server :)