Add information to error message.

PiperOrigin-RevId: 455171224

Author: Guy Nicholson

tensorflow_model_optimization/python/core/quantization/keras/quantize.py

@@ -441,12 +441,12 @@ def _quantize(layer):  # pylint: disable=missing-docstring
   # modifications don't affect the original model, or its weights.
   try:
     model_copy = _clone_model_with_weights(model)
-  except ValueError:
+  except ValueError as er:
     raise ValueError(
         'Unable to clone model. This generally happens if you used custom '
         'Keras layers or objects in your model. Please specify them via '
         '`quantize_scope` for your calls to `quantize_model` and '
-        '`quantize_apply`.')
+        '`quantize_apply`. [%s].' % er)
 
   # 2. Remove QuantizeAnnotate wrappers from the layers in the model. This
   # extracts the original model structure (easier to transform), and

tensorflow_model_optimization/python/core/quantization/keras/quantize_test.py

@@ -362,7 +362,7 @@ def testQuantizeCustomLayerWithoutQuantizeScope_RaisesError(self):
         '`quantize_scope` for your calls to `quantize_model` and '
         '`quantize_apply`.')
 
-    self.assertEqual(str(err.exception), expected_error)
+    self.assertIn(expected_error, str(err.exception))
 
   def testQuantize_RaisesErrorIfNoQuantizeConfig(self):
     annotated_model = keras.Sequential([