message: [tf.data service] Improve error message when non-fault-tolerant dispatcher restarts.
diff: @@ -654,7 +654,14 @@ Status DataServiceDispatcherImpl::GetTasks(const GetTasksRequest* request,
  mutex_lock l(mu_);
  VLOG(3) << "Looking up tasks for job client id " << request->job_client_id();
  std::shared_ptr<const Job> job;
  - TF_RETURN_IF_ERROR(state_.JobForJobClientId(request->job_client_id(), job));
  + Status s = state_.JobForJobClientId(request->job_client_id(), job);
  + if (errors::IsNotFound(s) && !config_.fault_tolerant_mode()) {
  + return errors::NotFound(
  + "Unknown job client id ", request->job_client_id(),
  + ". The dispatcher is not configured to be fault tolerant, so this "
  + "could be caused by a dispatcher restart.");
  + }
  + TF_RETURN_IF_ERROR(s);
  std::vector<std::shared_ptr<const Task>> tasks;
  TF_RETURN_IF_ERROR(state_.TasksForJob(job->job_id, tasks));
  for (const auto& task : tasks) {
  