More failed handling improvements.

Histoy manager now logs failed downloads same as it logs snatches and completed downloads.
Successfully downloaded shows are now removed from the failed.db history table allowing you to redownload same release.
failed.findRelease() now returns release name and provider name in a tuple.
failed.findRelease() fixed to properly clear failed.db history table of multi-snatches and incomplete release records and returns None if no release is found.