/**
  * @param object $command
  * @param Exception $exception
  * @return string|null
  */
 public function commandFailed($command, Exception $exception)
 {
     $exceptionClass = get_class($exception);
     $exceptionMessage = $exception->getMessage();
     return 'Command failed: ' . get_class($command) . ' ' . $this->serializer->encode($command) . " threw the exception {$exceptionClass} ({$exceptionMessage})";
 }