<!-- 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.
std::string label = LabelFromValue(request.params[0]);
    
    UniValue ret(UniValue::VOBJ);
    for (const std::pair<const CTxDestination, CAddressBookData>& item : pwallet->mapAddressBook) {
        if (item.second.name == label) {
            ret.pushKV(EncodeDestination(item.first), AddressBookDataToJSON(item.second, false));
        }
    }
    if (ret.empty()) {
        throw JSONRPCError(RPC_WALLET_INVALID_LABEL_NAME, std::string("No addresses with label " + label));
    }
    return ret;
}
static UniValue listlabels(const JSONRPCRequest& request)
{
    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)
        throw std::runtime_error(
            "listlabels ( \"purpose\" )\n"
            "\nReturns the list of all labels, or labels that are assigned to addresses with a specific purpose.\n"
            "\nArguments:\n"
            "1. \"purpose\"    (string, optional) Address purpose to list labels for ('send','receive'). An empty string is the same as not providing this argument.\n"
            "\nResult:\n"
            "[               (json array of string)\n"
            "  \"label\",      (string) Label name\n"
            "  ...\n"