Ensure In-Use Blocks Aren't Removed

[TheDiscordian]

#38 was solved by calculating the CID of a file, then removing all the blocks related to that file upon error, then (if necessary), re-adding the file. I believe this shouldn't be an issue as files are re-added, but I don't think they're re-added in every scenario. Either way, this could be a tricky problem to solve, and I'm opening this issue because it likely isn't solved.