return empty strings on openai error (#173)

                except openai.error.RateLimitError: # Server returned ratelimit response

                    ratelimit_attempt += 1

                    if ratelimit_attempt >= 3:

                        raise

                        new_translations = [''] * num_src

                        break

                    self.logger.warn(f'Restarting request due to ratelimiting by openai servers. Attempt: {ratelimit_attempt}')

                    time.sleep(2)

                except openai.error.APIError: # Server returned 500 error (probably server load)

                    retry_attempt += 1

                    if retry_attempt >= self.retry_attempts:

                        self.logger.error('OpenAI encountered a server error, possibly due to high server load. Use a different translator or try again later.')

                        raise

                        new_translations = [''] * num_src

                        break

                    self.logger.warn(f'Restarting request due to a server error. Attempt: {retry_attempt}')

                    time.sleep(1)

                except openai.error.ServiceUnavailableError:

                    retry_attempt += 1

                    if retry_attempt >= self.retry_attempts:

                        self.logger.error('OpenAI encountered a server error, possibly due to high server load. Use a different translator or try again later.')

                        new_translations = [''] * num_src

                        break

                    self.logger.warn(f'Restarting request due to a server error. Attempt: {retry_attempt}')

                    time.sleep(2)

                except InvalidNumTranslations:

                    retry_attempt += 1

                    message = f'number of translations does not match to source:\nprompt:\n    {prompt}\ntranslations:\n  {new_translations}\nopenai response:\n  {response}'