Merge bitcoin/bitcoin#30212: rename TransactionError:ALREADY_IN_CHAIN

e9de0a76b9 doc: release note for 30212 (willcl-ark)
87b1880525 rpc: clarify ALREADY_IN_CHAIN rpc errors (willcl-ark)

Pull request description:

  Closes: #19363

  Renaming this error improves clarity around the returned error both internally and externally when a transactions' outputs are already found in the utxo set (`TransactionError::ALREADY_IN_CHAIN -> TransactionError::ALREADY_IN_UTXO_SET`)

ACKs for top commit:
  tdb3:
    ACK e9de0a76b9
  ismaelsadeeq:
    ACK e9de0a76b9
  ryanofsky:
    Code review ACK e9de0a76b9.

Tree-SHA512: 7d2617200909790340951fe56a241448f9ce511900777cb2a712e8b9c0778a27d1f912b460f82335844224f1abb4322bc898ca076440959edade55c082a09237

src/node/transaction.cpp

@@ -55,7 +55,7 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
             const Coin& existingCoin = view.AccessCoin(COutPoint(txid, o));
             // IsSpent doesn't mean the coin is spent, it means the output doesn't exist.
             // So if the output does exist, then this transaction exists in the chain.
-            if (!existingCoin.IsSpent()) return TransactionError::ALREADY_IN_CHAIN;
+            if (!existingCoin.IsSpent()) return TransactionError::ALREADY_IN_UTXO_SET;
         }
 
         if (auto mempool_tx = node.mempool->get(txid); mempool_tx) {

src/node/types.h

@@ -19,7 +19,7 @@ namespace node {
 enum class TransactionError {
     OK, //!< No error
     MISSING_INPUTS,
-    ALREADY_IN_CHAIN,
+    ALREADY_IN_UTXO_SET,
     MEMPOOL_REJECTED,
     MEMPOOL_ERROR,
     MAX_FEE_EXCEEDED,