Fix portlet assignment

This fix makes page work even if a portlet that was provided by a third party add-on is no longer available, and logs about such problems into the event log.

After removing an add-on we got this error in one of the website pages: 

```
...
File "/opt/plone/buildout-cache/eggs/Solgema.PortletsManager-0.7-py2.7.egg/Solgema/PortletsManager/portlets/retriever.py", line 103, in getPortlets
assigned = ISolgemaPortletAssignment(assignment)
TypeError: ('Could not adapt', <persistent broken collective.portlet.adsense.adsenseportlet.Assignment instance '\x00\x00\x00\x00\x00(\xa8\x98'>, <InterfaceClass Solgema.PortletsManager.interfaces.ISolgemaPortletAssignment>)
```

To fix it I used a similar approach used in https://github.com/vipod/collective.braveportletsmanager to fix https://dev.plone.org/ticket/7375 but without the 'delete portlet' part.