This change better separates the different errors that can occur during an
upload. It does this by:
- Separating the authorization error into its own enum and having a variant
simply for that
- The `PacketLine` error was never constructed, so the error returned from the
`upload_pack::pktline::git_request` is now wrapped in this variant.
- The error from `upload_pack::upload_pack` is now wrapped in an `UploadPack`
variant.
- The `Io` variant is removed.
- Other redundant variants are also removed.
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