TX_MISSING_INPUTS is ambiguous

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.

Start by locating the TX_MISSING_INPUTS definition and the transaction-validation path that emits it. Review how missing and already-spent TXOs are distinguished, then clarify the error wording and determine whether separate errors are warranted; done means the reported cause no longer implies that inputs were omitted from the transaction.

Written by the indexing model from the issue text.