wallet: improve post-migration logging

Right now, after migration the last message users see is "migration completed",
but the migration isn't actually finished yet. We still need to load the new wallets
to ensure consistency, and if that fails, the migration will be rolled back. This
can be confusing for users.

This change logs the post-migration loading step and if a wallet fails to load and
the migration will be rolled back.

Github-Pull: bitcoin/bitcoin#34156
Rebased-From: d70b159c42

src/wallet/wallet.cpp

@@ -4463,7 +4463,12 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
         std::string name = to_reload->GetName();
         to_reload.reset();
         to_reload = LoadWallet(context, name, /*load_on_start=*/std::nullopt, options, status, error, warnings);
-        return to_reload != nullptr;
+        if (!to_reload) {
+            LogError("Failed to load wallet '%s' after migration. Rolling back migration to preserve consistency. "
+                     "Error cause: %s\n", wallet_name, error.original);
+            return false;
+        }
+        return true;
     };
 
     // Before anything else, check if there is something to migrate.
@@ -4544,6 +4549,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
     if (success) {
         // Migration successful, unload all wallets locally, then reload them.
         // Reload the main wallet
+        LogInfo("Loading new wallets after migration...\n");
         track_for_cleanup(*local_wallet);
         success = reload_wallet(local_wallet);
         res.wallet = local_wallet;