fix for non-annotatable objects

this fixes an error triggered when calling the comments viewlet's `get_replies` on an object that does not provide `IAnnotatable`.  using the mailhost here is a bit crude — in "real life" it was caused with ZWiki instances:

```python
Traceback (innermost last):
  ...
  Module plone.app.discussion.browser.comments, line 361, in get_replies
TypeError: ('Could not adapt', <ZWikiPage 'FrontPage' at 0xde18cd0>, <InterfaceClass plone.app.discussion.interfaces.IConversation>)
```