message: Log error in _WrappedSession.close
diff: @@ -1071,8 +1071,10 @@ class _WrappedSession(object):
  if self._sess:
  try:
  self._sess.close()
  - except _PREEMPTION_ERRORS:
  - pass
  + except _PREEMPTION_ERRORS as e:
  + logging.warning('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)
  finally:
  self._sess = None
  