(Yaf >=1.0.0)
Yaf_Exception_LoadFailed is thrown when the loader
fails to load a required script. Its error code is
520. The more specific failures are represented by
its subclasses Yaf_Exception_LoadFailed_Module,
Yaf_Exception_LoadFailed_Controller,
Yaf_Exception_LoadFailed_Action and
Yaf_Exception_LoadFailed_View.