message: Clarify error message when importing a GraphDef with unknown ops.
  This should make the situation from github.com/tensorflow/tensorflow/issues/17014 less confusing.
diff: @@ -572,7 +572,14 @@ def import_graph_def(graph_def,
  if node.name in name_to_op:
  raise ValueError('Duplicate name \'%s\' in GraphDef.' % node.name)
  if node.op not in op_dict:
  - raise ValueError('No op named %s in defined operations.' % node.op)
  + raise ValueError(
  + 'No op named %s in defined operations. If the Graph you are '
  + 'importing uses custom ops or any parts of tf.contrib, you '
  + 'should explicitly import the libraries defining those ops '
  + 'before loading the Graph. Note that tf.contrib is lazily loaded '
  + 'when accessed, so simply referencing (e.g.) '
  + '`tf.contrib.resampler` will cause those ops to be made '
  + 'available.' % node.op)
  op_def = op_dict[node.op]
  output_types = _OutputTypes(node, op_dict)
  