Fixed an ElasticSearch error when running `rebuild_index`

When using ElasticSearch as the search engine you get a strange 404 error. This is because the backend is trying to refresh the index after it has deleted the index.

```
$ bin/manage rebuild_index
WARNING: This will irreparably remove EVERYTHING from your search index in connection 'default'.
Your choices after this are to restore from backups or rebuild via the `rebuild_index` command.
Are you sure you wish to continue? [y/N] y

Removing all documents from your index because you said so.
Failed to clear Elasticsearch index: Non-OK status code returned (404) containing u'IndexMissingException[[my_index] missing]'.
All documents removed.
Indexing 40 components.
```