Count error document to complete adding documents to the database.

Previously DocumentLoader#LoaderTask had a counter to count loaded
documents and completes adding documents to the database.  However it
does not count documents where a MTP device returns an error for
getObjectInfo. The CL fixes the problem to ensure we complete documents
loading.

BUG=27729653

Change-Id: I696eac790a6535f1bd7a1855dc2d6f932e32eae5