Partially revert PR #2190

The core shouldn't depend on constants that are not defined by itself
diff --git a/system/libraries/Driver.php b/system/libraries/Driver.php
index d2e41d6..d15d34f 100644
--- a/system/libraries/Driver.php
+++ b/system/libraries/Driver.php
@@ -290,7 +290,7 @@
 
 		$trace = debug_backtrace();
 		_exception_handler(E_ERROR, "No such method '{$method}'", $trace[1]['file'], $trace[1]['line']);
-		exit(EXIT_UNKNOWN_METHOD);
+		exit(6); // EXIT_UNKNOWN_METHOD
 	}
 
 	// --------------------------------------------------------------------