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.

Rebased