Support nested error handler

Added support for nested `Zend\Stdlib\ErrorHandler` to make the use of the error handler more simple

 - added `Zend\Stdlib\ErrorHandler::getNestedLevel()`
 - added `Zend\Stdlib\ErrorHandler::clean()` to stop all active handler
 - `Zend\Stdlib\ErrorHandler::start()` no longer throws an exception if already started
 - `Zend\Stdlib\ErrorHandler::stop()` no longer throws an exception if not started
 - It's backward compatible (if not using an own extended error handler)