Update HTTP exceptions

We need to replace the `Http\Client\Exception\HttpException` in `IBM\Watson\Core\Client\ExceptionHandler:handle` with a more specific exception.

Watson error responses are usually in the format:

```json
{
"error": "General description of an error.",
"errors": [
{
"message": "Description of a specific constraint violation.",
"path": "The location of the constraint violation."
}
],
"code": 500
}
```