Return a status enum from ScanForWalletTransactions

Return the failed block as an out var.

This clarifies the outcome as the prior return value could
be null due to user abort or failure.

src/qt/test/wallettests.cpp

@@ -146,7 +146,12 @@ void TestGUI()
         auto locked_chain = wallet->chain().lock();
         WalletRescanReserver reserver(wallet.get());
         reserver.reserve();
-        wallet->ScanForWalletTransactions(chainActive.Genesis(), nullptr, reserver, true);
+        const CBlockIndex* const null_block = nullptr;
+        const CBlockIndex* stop_block;
+        QCOMPARE(
+            wallet->ScanForWalletTransactions(chainActive.Genesis(), nullptr, reserver, stop_block, true /* fUpdate */),
+            CWallet::ScanResult::SUCCESS);
+        QCOMPARE(stop_block, null_block);
     }
     wallet->SetBroadcastTransactions(true);