Attachments from now-deleted nodes cause exceptions when rendering results

When search results include files attached to entities which no longer exist (in my case, nodes which have been deleted but not yet cleared from the index), theme_apachesolr_search_snippets__file's use of entity_extract_ids raises an exception which gets caught and handled as a failure to communicate with the search engine.

I'm honestly not sure if this is a bug or behaving as designed and there's a bug elsewhere. In any case, I'll attach a patch which attempts to handle the exception nicely.