maybe_download_addons() treated an addon as already downloaded whenever its
directory existed. A download that fails partway leaves an empty directory
behind, which is then trusted on every later launch, so confirm_paths()
raises InvalidAddonPath: manifest.json is missing and never recovers. Gate
the check on manifest.json presence and rmtree the partial directory on
failure. Closes#308.
(cherry picked from commit 0a8211969b)
