message: Change the logging from warning to error when there is an exception at session close.
diff: @@ -1189,7 +1189,7 @@ class _WrappedSession(object):
  try:
  self._sess.close()
  except _PREEMPTION_ERRORS as e:
  - logging.warning(
  + logging.error(
  'An error occurred when attempting to close the '
  'session. This may be due to a preemption in a '
  'connected worker or parameter server. Error: %s', e)
  