Partially revert PR #2190

The core shouldn't depend on constants that are not defined by itself
diff --git a/system/core/Exceptions.php b/system/core/Exceptions.php
index 54a5bc4..0418696 100644
--- a/system/core/Exceptions.php
+++ b/system/core/Exceptions.php
@@ -125,7 +125,7 @@
 		}
 
 		echo $this->show_error($heading, $message, 'error_404', 404);
-		exit(EXIT_UNKNOWN_FILE);
+		exit(4); // EXIT_UNKNOWN_FILE
 	}
 
 	// --------------------------------------------------------------------