<!-- 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.
#include <wallet/feebumper.h>
#include <wallet/rpcwallet.h>
#include <wallet/wallet.h>
#include <wallet/walletdb.h>
#include <wallet/walletutil.h>
#include <stdint.h>
#include <univalue.h>
#include <functional>
static const std::string WALLET_ENDPOINT_BASE = "/wallet/";
bool GetWalletNameFromJSONRPCRequest(const JSONRPCRequest& request, std::string& wallet_name)
{
    if (request.URI.substr(0, WALLET_ENDPOINT_BASE.size()) == WALLET_ENDPOINT_BASE) {
        
        wallet_name = urlDecode(request.URI.substr(WALLET_ENDPOINT_BASE.size()));
        return true;
    }
    return false;
}
std::shared_ptr<CWallet> GetWalletForJSONRPCRequest(const JSONRPCRequest& request)
{
    std::string wallet_name;
    if (GetWalletNameFromJSONRPCRequest(request, wallet_name)) {
        std::shared_ptr<CWallet> pwallet = GetWallet(wallet_name);
        if (!pwallet) throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Requested wallet does not exist or is not loaded");
        return pwallet;
    }
    std::vector<std::shared_ptr<CWallet>> wallets = GetWallets();
    return wallets.size() == 1 || (request.fHelp && wallets.size() > 0) ? wallets[0] : nullptr;
}
std::string HelpRequiringPassphrase(CWallet * const pwallet)