ZF-5978: Logger sanity check workflow has to be improved
Logger sanity check is performed at each Zend_Cache_Backend::setDirectives() call. So it's done at each logger object update. If logging directive is set but existing logger is not an instance of Zend_Log then default logger is created instead instead of throwing exception.
_log() method has a error. It throws an exception if "logger is not set" *or* if logger *is* an instance of Zend_Log.