message: improved estimator.export_savedmodel exception
diff: @@ -191,7 +191,8 @@ def build_all_signature_defs(receiver_tensors,
  if not isinstance(receiver_tensors, dict):
  receiver_tensors = {_SINGLE_RECEIVER_DEFAULT_NAME: receiver_tensors}
  if export_outputs is None or not isinstance(export_outputs, dict):
  - raise ValueError('export_outputs must be a dict.')
  + raise ValueError('export_outputs must be a dict and not'
  + '{}'.format(type(export_outputs)))
  signature_def_map = {}
  excluded_signatures = {}
  