dtoc: Improve internal error for Refresh()

Add the node name too so it is easy to see which node failed.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test_fdt.py b/tools/dtoc/test_fdt.py
index 1c3a8a2..72095b0 100755
--- a/tools/dtoc/test_fdt.py
+++ b/tools/dtoc/test_fdt.py
@@ -209,7 +209,7 @@
         del self.node.props['notstring']
         with self.assertRaises(ValueError) as e:
             self.dtb.Refresh()
-        self.assertIn("Internal error, property 'notstring' missing, offset ",
+        self.assertIn("Internal error, node '/spl-test' property 'notstring' missing, offset ",
                       str(e.exception))
 
     def testLookupPhandle(self):