Fix offline exceptions when trying to resolve remote index.

* Remove repository/ from gitignore, so that package still shows up.
* Add RepositoryException which is thrown on resolution issues (sort of)
* Specifically catch repository exception issues on acto startup, so that
  we make it to a clean 'started' state, even if we can't find the latest
  repo.
* We still bomb and die if there was no pre-seeded local repository.

Review by @havocp