code: public function fault($exception = null, $code = null)
      {
          // Copié collé de Zend_Server_Rest::fault() ...
          if (isset($this->_functions[$this->_method]))
          {
              $function = $this->_functions[$this->_method];
          }
          elseif (isset($this->_method))
          {
              $function = $this->_method;
          }
          else
          {
              $function = 'rest';
          }
  
          if ($function instanceof Zend_Server_Reflection_Method)
          {
              $class = $function->getDeclaringClass()->getName();
          }
          else
          {
              $class = false;
          }
  
          if ($function instanceof Zend_Server_Reflection_Function_Abstract)
          {
              $method = $function->getName();
          }
          else
          {
              $method = $function;
          }
  
          // Message et code d'erreur
          $exception_message = null;
          $code_message = null;
          
          if($exception instanceof Exception)
          {
              $code_message = $exception->getCode();
              $exception_message = $exception->getMessage();
          }
          elseif(($exception !== null) || 'rest' == $function)
          {
              $exception_message = 'An unknown error occured. Please try again.';
          }
          else
          {
              $exception_message = 'Call to ' . $method . ' failed.';
          }
  
          // Headers to send
          if ($code_message === null || (404 != $code_message))
          {
              $this->_headers[] = 'HTTP/1.0 400 Bad Request';
          }
          else
          {
              $this->_headers[] = 'HTTP/1.0 404 File Not Found';
          }
          
          // On envoie le message d'erreur
          return $exception_message;
      }
docs: Implement Zend_Server_Interface::fault()
  
  Creates XML error response, returning DOMDocument with response.
  
  @param string|Exception $fault Message
  @param int $code Error Code
  @return DOMDocument