message: Update error message upon a preemption error to highlight a potential
  gRPC failure and suggest increasing the number of parameter servers.
diff: @@ -1114,7 +1114,11 @@ class _RecoverableSession(_WrappedSession):
  logging.info('An error was raised while a session was being created. '
  'This may be due to a preemption of a connected worker '
  'or parameter server. A new session will be created. '
  - 'Error: %s', e)
  + 'This error may also occur due to a gRPC failure caused '
  + 'by high memory or network bandwidth usage in the '
  + 'parameter servers. If this error occurs repeatedly, try '
  + 'increasing the number of parameter servers assigned to '
  + 'the job. Error: %s', e)
  def _check_stop(self):
  try:
  @@ -1127,7 +1131,11 @@ class _RecoverableSession(_WrappedSession):
  'session is complete. This may be due to a preemption in '
  'a connected worker or parameter server. The current '
  'session will be closed and a new session will be '
  - 'created. Error: %s', e)
  + 'created. This error may also occur due to a gRPC failure '
  + 'caused by high memory or network bandwidth usage in the '
  + 'parameter servers. If this error occurs repeatedly, try '
  + 'increasing the number of parameter servers assigned to '
  + 'the job. Error: %s', e)
  self.close()
  self._sess = self._create_session()
  # Since we have just recreated the session, the overall computation should
  @@ -1150,7 +1158,11 @@ class _RecoverableSession(_WrappedSession):
  logging.info('An error was raised. This may be due to a preemption in '
  'a connected worker or parameter server. The current '
  'session will be closed and a new session will be '
  - 'created. Error: %s', e)
  + 'created. This error may also occur due to a gRPC failure '
  + 'caused by high memory or network bandwidth usage in the '
  + 'parameter servers. If this error occurs repeatedly, try '
  + 'increasing the number of parameter servers assigned to '
  + 'the job. Error: %s', e)
  self.close()
  self._sess = None
  @@ -1166,7 +1178,11 @@ class _RecoverableSession(_WrappedSession):
  logging.info('An error was raised. This may be due to a preemption in '
  'a connected worker or parameter server. The current '
  'session will be closed and a new session will be '
  - 'created. Error: %s', e)
  + 'created. This error may also occur due to a gRPC failure '
  + 'caused by high memory or network bandwidth usage in the '
  + 'parameter servers. If this error occurs repeatedly, try '
  + 'increasing the number of parameter servers assigned to '
  + 'the job. Error: %s', e)
  self.close()
  self._sess = None
  