<!-- Describe the issue -->

When trying to sign a transaction in a encrypted wallet that is locked, version 0.16 would return the following error:

```"Please enter the wallet passphrase with walletpassphrase first."```

While doing exactly the same procedure against 0.17, the response is something like:

```json
"hex": "020000000183ac9cc1137c444f32356fb070ecba194c87a05e43674c06a55f5b1d0561eabc0000000000ffffffff0180626a94000000001600141875aa6ab19c2551c11759cf3e9f4dd7aa40bcf500000000",
       "complete": false,
       "errors": [
         {
           "txid": "bcea61051d5b5fa5064c67435ea0874c19baec70b06f35324f447c13c19cac83",
           "vout": 0,
           "witness": [
           ],
           "scriptSig": "",
           "sequence": 4294967295,
           "error": "Unable to sign input, invalid stack size (possibly missing key)"
         }
     ]

```

<!--- What behavior did you expect? -->

v0.17 should check if the wallet is locked before trying to sign the tx and return the same error as v0.16 for consistency reasons.
+ HelpExampleRpc("loadwallet", "\"test.dat\"")
        );
    std::string wallet_file = request.params[0].get_str();
    std::string error;
    fs::path wallet_path = fs::absolute(wallet_file, GetWalletDir());
    if (fs::symlink_status(wallet_path).type() == fs::file_not_found) {
        throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Wallet " + wallet_file + " not found.");
    } else if (fs::is_directory(wallet_path)) {
        
        fs::path wallet_dat_file = wallet_path / "wallet.dat";
        if (fs::symlink_status(wallet_dat_file).type() == fs::file_not_found) {
            throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Directory " + wallet_file + " does not contain a wallet.dat file.");
        }
    }
    std::string warning;
    if (!CWallet::Verify(wallet_file, false, error, warning)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Wallet file verification failed: " + error);
    }
    std::shared_ptr<CWallet> const wallet = CWallet::CreateWalletFromFile(wallet_file, fs::absolute(wallet_file, GetWalletDir()));
    if (!wallet) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Wallet loading failed.");
    }
    AddWallet(wallet);
    wallet->postInitProcess();
    UniValue obj(UniValue::VOBJ);
    obj.pushKV("name", wallet->GetName());
    obj.pushKV("warning", warning);
    return obj;
}
static UniValue createwallet(const JSONRPCRequest& request)