MessageFormatter - check if response is set

I was getting `Call to a member function getInfo() on a non-object in Guzzle/Log/MessageFormatter.php on line 149` error when using BackoffPlugin with backoff formatter (default message format) and curl error was thrown.

See more in stack trace: https://travis-ci.org/keboola/storage-api-php-client/builds/7421542

Guzzle and plugins setup: https://github.com/keboola/storage-api-php-client/blob/5beab6c4498480886c4f609e10c75697a4a31acd/src/Keboola/StorageApi/Client.php#L122