display more error details when postStream failed

[sion-codin]

when use a deactivated account's api key to createStream

    final chatStream = OpenAI.instance.chat.createStream(
      model: "gpt-3.5-turbo",
      temperature: temperature,
      messages: messages,
    );

openai backend will return:

{
    "message":"",
    "type":"invalid_request_error",
    "param":null,
    "code":"account_deactivated"
}

Note: the message is empty.

in this lib, RequestFailedException.message will empty, the developer can only obtain the network request status code :

                if (doesErrorExists(decodedData)) {
                  final error = decodedData[OpenAIStrings.errorFieldKey]
                      as Map<String, dynamic>;
                  final message = error[OpenAIStrings.messageFieldKey];
                  final statusCode = respond.statusCode;
                  final exception = RequestFailedException(message, statusCode);

                  controller.addError(exception);
                }

This PR does the following:

final message = error[OpenAIStrings.messageFieldKey];

change to

final message = jsonEncode(error);

[anasfik]

Thank you for this notice, and yes this is helpful for the developer but is also a breaking change, can we make a conditional expression to check whether the message field is empty or not, if yes then we apply your implementation, otherwise we return directly that message field.

What do you think?

[sion-codin]

I am a PR novice 😂.If there is anything incorrect here, please guide me. thanks

[anasfik]

Thank you