Adds Error variants for a couple of spentbook errors.
It's a bit gross to put them sn_dbc::Error, however I'm thinking it will be split up soon when we move to a modules approach for MintNode, Wallet, Spentbook. So this is temporary.
fix: return Error::SpentbookKeyImageAlreadySpent instead of panic
fix: return Error::SpentbookRingSizeMismatch instead of assert failure
Adds Error variants for a couple of spentbook errors.
It's a bit gross to put them sn_dbc::Error, however I'm thinking it will be split up soon when we move to a modules approach for MintNode, Wallet, Spentbook. So this is temporary.