Caching and fail support updated. (See description, important.)

Safe caching implemented and fail support updated. Control of fail support had been moved into backlog, providing updating of parsed entries and leaving further handling to backlog as usual. Most functions and known issues have been addressed. The primary remaining concern is not internal to parse_urls but the general behavior of backlogging all entries of a task in case of an unhandled error, rather than accepted (and failed) entries only, leading to unfortunate reparsing.