<!-- 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::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() > 2)
    {
        std::string msg = "addwitnessaddress \"address\" ( p2sh )\n"
            "\nDEPRECATED: set the address_type argument of getnewaddress, or option -addresstype=[bech32|p2sh-segwit] instead.\n"
            "Add a witness address for a script (with pubkey or redeemscript known). Requires a new wallet backup.\n"
            "It returns the witness script.\n"
            "\nArguments:\n"
            "1. \"address\"       (string, required) An address known to the wallet\n"
            "2. p2sh            (bool, optional, default=true) Embed inside P2SH\n"
            "\nResult:\n"
            "\"witnessaddress\",  (string) The value of the new address (P2SH or BIP173).\n"
            "}\n"
        ;
        throw std::runtime_error(msg);
    }
    if (!IsDeprecatedRPCEnabled("addwitnessaddress")) {
        throw JSONRPCError(RPC_METHOD_DEPRECATED, "addwitnessaddress is deprecated and will be fully removed in v0.17. "
            "To use addwitnessaddress in v0.16, restart bitcoind with -deprecatedrpc=addwitnessaddress.\n"
            "Projects should transition to using the address_type argument of getnewaddress, or option -addresstype=[bech32|p2sh-segwit] instead.\n");
    }
    CTxDestination dest = DecodeDestination(request.params[0].get_str());
    if (!IsValidDestination(dest)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
    }