TX_MISSING_INPUTS is ambiguous

[BrannonKing]

The TX_MISSING_INPUTS error makes it sound like the person creating the TX forgot to include inputs in that TX. However, the error is actually issued when the TX's inputs don't have corresponding TXOs in the history of the chain or if the TXOs have already been spent. We should clarify the message and consider breaking it into spent vs missing.

[BrannonKing]

Should the error state what TXIs were already spent? Or should there be a way to query via RPC for what was spent? Maybe the unspent Sqlite table is sufficient for that.