FetchTxBySha changes what it returns, it can now return a TxListReply and
and error if none are found.
FetchTxByShaList is renamed to FetchUnSpentTxByShaList to indicate that
it will (likey/eventually) only return Tx that have some unspent TxOuts.
Tx which are fully spent may not be (reliably) looked up using this API.
Fix error returns in InsertBlock and FetchBlockBySha
Give up on return by name in InsertBlock() and return explicit
err one location in FetchBlockBySha to return proper error value
