message: better error message for undefined inputs
diff: // ATTENTION: The code in this file is highly EXPERIMENTAL.
  // Adventurous users should note that the APIs will probably change.
  +#include <sstream>
  #include "onnx/common/ir_pb_converter.h"
  namespace ONNX_NAMESPACE {
  @@ -281,6 +282,11 @@ std::unique_ptr<Graph> graphProtoToGraph(
  value_by_name_of[input] = undef->outputs()[0];
  }
  + if (!value_by_name_of.count(input)) {
  + std::ostringstream msg;
  + msg << "Input " << input << " is undefined!";
  + throw std::out_of_range(msg.str());
  + }
  n->addInput(value_by_name_of.at(input));
  }
  }
  