How Can I show a custom view instead of the generic error attached in cases of the database going down unexpectedly?

In applications folder there is a folder named errors. There are all error files which comes up when an error is occurred.
When database error is occured, error_db.php file is shown in browser. You can edit it or replace that with your custom file.
Hope this helps.
application/views folder it means you are using CodeIgniter version 2.x. It will be available in CI 3.x versions.