ApiError - keep complete response

...for easier response type check by code like this:

```ruby
rescue SUSE::Connect::ApiError => e
  case e.response
  when Net::HTTPClientError
    ...
  when Net::HTTPServerError
     ...
  end
end
```

It allows to use descriptive class names instead of number constants (`Net::HTTPNotFound` instead of `404`) or ranges (`Net::HTTPClientError` instead of `400..499`), the code is then more readable.