On GUI side, handle Core REST API responses without data

[kozlovsky]

This PR fixes #7333:

1. It adds the handling of QNeworkReply errors mentioned in Empty results in reply.readAll() when receiving response from Core REST API #7335 by checking the QNetworkReply error code before reading the response data;
2. If there is an error or if the response data is empty, now Request does not call the on_success callback (renamed from on_finish);
3. In case of an error, it displays QNetworkReply errors in debug window as a request status code.

[xoriole]

Related to #7558

By checking the QNetworkReply error code before reading the response data, I noticed that it is masking the HTTP error response like 413 HTTPRequestEntityTooLarge because the error code according to QT is not zero in such case. It returns error code: 299 (UnknownContentError).

Therefore, in my opinion, it would be best to check for the HTTP status code first and process the response (including empty and error responses). If there is no HTTP status code present then the QT error code be processed.

[kozlovsky]

@xoriole, in my opinion, we should check the QNetworkReply error code before the HTTP error code because it covers a wider range of possible errors, like QNetworkReply::ConnectionRefusedError or QNetworkReply::UnknownNetworkError. It does not look correct to check the HTTP code if no HTTP response was received.

But we should allow further processing of responses with QNetworkReply::UnknownContentError, as in this case, we successfully got the response from the server, and the error is content-related and most probably explained by the HTTP error code and JSON error message.

So I think we should change

if error_code != QNetworkReply.NoError:
    ...

to

if error_code not in (QNetworkReply.NoError, QNetworkReply.UnknownContentError):
    ...

and allow processing of responses with QNetworkReply.UnknownContentError.