Handle Cloudsearch Indexing Errors

Previously the code handled errors on individual items but not an error response that included no `adds` or `deletes` keys resulting in a KeyError.

For example with this patch means that you get an exception like:

    boto.cloudsearch2.document.SearchServiceException: Error indexing documents => User: anonymous is not authorized to perform: cloudsearch:document on resource...

Rather than:

    KeyError: 'adds'