use executeAudited in files cache

This PR replaces all DB queries in the file cache with th OC_DB::executeAudited wrapper that checks for a database error and throws an exception that is logged and shown to the user. It will also show up in the phpunit tests and make db errors in the filecache much easier to spot.

@icewind1991 @DeepDiver1975 @karlitschek @MTGap @bartv2 @Raydiation @kabum