By default detail error reporting is turned on. If somehow this feature is turned off, when there is a problem in a page, a friendly error message will be displayed. If you are developing it is helpful to see detailed error information which can be useful for debugging and troubleshooting issues.
To ensure the friendly error messages are not displayed, open the config file (config/config.php) and locate the property SHOW_FRIENDLY_ERRORS and set this to ‘false’.
$CONFIG['SHOW_FRIENDLY_ERRORS'] = false;
When development has ended, it is advised you enable friendly errors again to prevent users from seeing information about your site.
Alternative Error Tracking
Most site errors are also available in other locations. On the file system you can find a log file (logs/error.txt) which contains recent errors. They are also stored in the database and can be accessed via the admin area by selecting Settings \> Error Log.
Modifying the friendly error page
If you wish to customise the friendly error page, you can do this by modifying the HTML file located under /modules/cms/error_pages/error.htm