categorical accessor (and others?) should fail if an unknown attribute is set

[janschulz]

In[10]: a = pandas.Series(pandas.Categorical(list("abc")))
In[11]: a.cat.labels = [1,2]
In[12]: a.cat.labels
Traceback (most recent call last):
  File "C:\portabel\miniconda\envs\pandas_dev\lib\site-packages\IPython\core\interactiveshell.py", line 2883, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-12-a68ee763e4e8>", line 1, in <module>
    a.cat.labels
AttributeError: 'CategoricalAccessor' object has no attribute 'labels'