Bugfix for concatenating node instead of str. (#1933)

Signed-off-by: Francesco Di Natale <frank.dinatale1988@gmail.com>

Co-authored-by: Jay Zhang <36183870+fatcat-z@users.noreply.github.com>

Author: FrankD412

tf2onnx/graph.py

@@ -751,10 +751,10 @@ def reset_nodes(self, ops):
 
         for n in self.inputs:
             if n not in ops:
-                raise ValueError("graph input " + n + " not exist")
+                raise ValueError("graph input '" + n.name + "' not exist")
         for o in self.outputs:
             if o not in self._output_to_node_name:
-                raise ValueError("graph output " + o + " not exist")
+                raise ValueError("graph output '" + o.name + "' not exist")
 
         self._dtypes = remained_dtypes
         self._output_shapes = remained_shapes