Add Error to Result added note for tokio

[sain-VO]

Added error to Result for main, otherwise it won't compile. If tokio isn't added with features = ["full"] main won't work, so I thought it would be a good idea to remind them.

fixes #ISSUE_ID

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[`Entry::unpack`]: https://docs.rs/tar/*/tar/struct.Entry.html#method.unpack

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