message: Add an error type in the logging of worker failure.
  Fix the error of:
  logging.error("Cluster update failed with error: %s. Retrying...", e)
  UnboundLocalError: local variable 'e' referenced before assignment
diff: @@ -673,7 +673,7 @@ class WorkerPreemptionHandler(object):
  return
  self._validate_preemption_failure(e)
  - logging.error("Worker %s failed with error: %s", worker_device_name, e)
  + logging.error("Worker %s failed with %r:%s", worker_device_name, e, e)
  if on_failure_fn:
  on_failure_fn()
  @@ -727,10 +727,10 @@ class WorkerPreemptionHandler(object):
  except Exception as e: # pylint: disable=broad-except
  try:
  self._validate_preemption_failure(e)
  - except Exception as e: # pylint: disable=broad-except
  + except Exception as ps_e: # pylint: disable=broad-except
  # In this case, a parameter server fails. So we raise this error to
  # the caller of `wait_on_failure`.
  - self._error_from_recovery = e
  + self._error_from_recovery = ps_e
  self._worker_up_cond.notify_all()
  if self._should_preemption_thread_run:
  self._cluster_due_for_update_or_finish.clear()
  