`ScanForWalletTransactions()` has the following comment about its return value:

```
 * Returns null if scan was successful. Otherwise, if a complete rescan was not
 * possible (due to pruning or corruption), returns pointer to the most recent
 * block that could not be scanned.
```

This is not strictly true. If `abortrescan` is called, then `ScanForWalletTransactions()` will return `nullptr`, even though the scan wasn't successful to the tip.

This in turn causes a minor bug in `importmulti`, where the call doesn't return the correct error and warning about potentially not finding transactions if `abortrescan` was called.

I think the correct fix is for `ScanForWalletTransactions()` to inform the caller that the rescan was aborted.
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 4)
        throw std::runtime_error(
            "walletprocesspsbt \"psbt\" ( sign \"sighashtype\" bip32derivs )\n"
            "\nUpdate a PSBT with input information from our wallet and then sign inputs\n"
            "that we can sign for.\n"
            + HelpRequiringPassphrase(pwallet) + "\n"
            "\nArguments:\n"
            "1. \"psbt\"                      (string, required) The transaction base64 string\n"
            "2. sign                          (boolean, optional, default=true) Also sign the transaction when updating\n"
            "3. \"sighashtype\"            (string, optional, default=ALL) The signature hash type to sign with if not specified by the PSBT. Must be one of\n"
            "       \"ALL\"\n"
            "       \"NONE\"\n"
            "       \"SINGLE\"\n"
            "       \"ALL|ANYONECANPAY\"\n"
            "       \"NONE|ANYONECANPAY\"\n"
            "       \"SINGLE|ANYONECANPAY\"\n"
            "4. bip32derivs                    (boolean, optional, default=false) If true, includes the BIP 32 derivation paths for public keys if we know them\n"