Partially revert PR #2190

The core shouldn't depend on constants that are not defined by itself
diff --git a/system/database/DB_driver.php b/system/database/DB_driver.php
index b004de3..fafce45 100644
--- a/system/database/DB_driver.php
+++ b/system/database/DB_driver.php
@@ -1676,7 +1676,7 @@
 
 		$error =& load_class('Exceptions', 'core');
 		echo $error->show_error($heading, $message, 'error_db');
-		exit(EXIT_DATABASE);
+		exit(8); // EXIT_DATABASE
 	}
 
 	// --------------------------------------------------------------------