refactor: interfaces, make 'createTransaction' less error-prone

Bundle all function's outputs inside the util::Result returned object.

Reasons for the refactoring:
- The 'change_pos' ref argument has been a source of bugs in the past.
- The 'fee' ref argument is currently only set when the transaction creation process succeeds.

src/qt/walletmodel.cpp

@@ -23,6 +23,7 @@
 #include <psbt.h>
 #include <util/translation.h>
 #include <wallet/coincontrol.h>
+#include <wallet/types.h>
 #include <wallet/wallet.h>
 
 #include <cstdint>
@@ -149,6 +150,8 @@ bool WalletModel::validateAddress(const QString& address) const
 
 WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransaction &transaction, const CCoinControl& coinControl)
 {
+    transaction.getWtx() = nullptr; // reset tx output
+
     CAmount total = 0;
     bool fSubtractFeeFromAmount = false;
     QList<SendCoinsRecipient> recipients = transaction.getRecipients();
@@ -199,22 +202,21 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
     }
 
     try {
-        CAmount nFeeRequired = 0;
-        int nChangePosRet = -1;
-
         auto& newTx = transaction.getWtx();
-        const auto& res = m_wallet->createTransaction(vecSend, coinControl, /*sign=*/!wallet().privateKeysDisabled(), nChangePosRet, nFeeRequired);
-        newTx = res ? *res : nullptr;
-        transaction.setTransactionFee(nFeeRequired);
-        if (fSubtractFeeFromAmount && newTx)
-            transaction.reassignAmounts(nChangePosRet);
-
-        if (!newTx) {
+        const auto& res = m_wallet->createTransaction(vecSend, coinControl, /*sign=*/!wallet().privateKeysDisabled(), /*change_pos=*/std::nullopt);
+        if (!res) {
             Q_EMIT message(tr("Send Coins"), QString::fromStdString(util::ErrorString(res).translated),
-                CClientUIInterface::MSG_ERROR);
+                           CClientUIInterface::MSG_ERROR);
             return TransactionCreationFailed;
         }
 
+        newTx = res->tx;
+        CAmount nFeeRequired = res->fee;
+        transaction.setTransactionFee(nFeeRequired);
+        if (fSubtractFeeFromAmount && newTx) {
+            transaction.reassignAmounts(static_cast<int>(res->change_pos.value_or(-1)));
+        }
+
         // Reject absurdly high fee. (This can never happen because the
         // wallet never creates transactions with fee greater than
         // m_default_max_tx_fee. This merely a belt-and-suspenders check).