code: private static ApiException createSyntheticErrorForRpcFailure(Throwable overallRequestError) {
      if (overallRequestError instanceof ApiException) {
        ApiException requestApiException = (ApiException) overallRequestError;
  
        return ApiExceptionFactory.createException(
            "Didn't receive a result for this mutation entry",
            overallRequestError,
            requestApiException.getStatusCode(),
            requestApiException.isRetryable());
      }
  
      return ApiExceptionFactory.createException(
          "Didn't receive a result for this mutation entry",
          overallRequestError,
          LOCAL_UNKNOWN_STATUS,
          false);
    }
docs: Create a synthetic {@link ApiException} for an individual entry. When the entire RPC fails, it
  implies that all entries failed as well. This helper is used to make that behavior explicit.
  The generated exception will have the overall error as its cause.