Wrong exception type raised on error 500

Received the following exception during `Emarsys::Contact.update_batch`
```
Emarsys::BadRequest
errorHTTP-Code: 500, Emarsys-Code: 1 - Database connection failed: timeout
```

Seems like it should be `Emarsys::InternalServerError` instead of `Emarsys::BadRequest` that should be raised right?

Gem version  (0.3.3)
