The document cache silently accepts a failed database load when it loads an existing document

When a database load fails without exception, the document cache stores the fact that this document doesn't exist anymore even if the exists cache previously indicated that it existed. This seems like an impossible scenario but documents randomly disappearing on xwiki.org suggest that it might actually be possible. XWiki should at least log an error when this happens so we can better understand why it happens.

During document deletion, there is a possible race condition where this state would be expected as the document is first removed from the cache and then from the exists cache. I couldn't reproduce this even when requesting the page of the document concurrently by 8 threads while deleting the document but I cannot exclude it so the logged message should indicate that this might not always be a bug.