message: update error checking on TensorFlow 2.0
  change ImportError to AttributeError.
diff: @@ -157,7 +157,7 @@ def main(_):
  if FLAGS.quantize:
  try:
  tf.contrib.quantize.create_training_graph(quant_delay=0)
  - except ImportError as e:
  + except AttributeError as e:
  msg = e.args[0]
  msg += ('\n\n The --quantize option still requires contrib, which is not '
  'part of TensorFlow 2.0. Please install a previous version:'
  