<!-- 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.
return true;
}
static UniValue getwalletinfo(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() != 0)
        throw std::runtime_error(
            "getwalletinfo\n"
            "Returns an object containing various wallet state info.\n"
            "\nResult:\n"
            "{\n"
            "  \"walletname\": xxxxx,               (string) the wallet name\n"
            "  \"walletversion\": xxxxx,            (numeric) the wallet version\n"
            "  \"balance\": xxxxxxx,                (numeric) the total confirmed balance of the wallet in " + CURRENCY_UNIT + "\n"
            "  \"unconfirmed_balance\": xxx,        (numeric) the total unconfirmed balance of the wallet in " + CURRENCY_UNIT + "\n"
            "  \"immature_balance\": xxxxxx,        (numeric) the total immature balance of the wallet in " + CURRENCY_UNIT + "\n"
            "  \"txcount\": xxxxxxx,                (numeric) the total number of transactions in the wallet\n"
            "  \"keypoololdest\": xxxxxx,           (numeric) the timestamp (seconds since Unix epoch) of the oldest pre-generated key in the key pool\n"
            "  \"keypoolsize\": xxxx,               (numeric) how many new keys are pre-generated (only counts external keys)\n"
            "  \"keypoolsize_hd_internal\": xxxx,   (numeric) how many new keys are pre-generated for internal use (used for change outputs, only appears if the wallet is using this feature, otherwise external keys are used)\n"
            "  \"unlocked_until\": ttt,             (numeric) the timestamp in seconds since epoch (midnight Jan 1 1970 GMT) that the wallet is unlocked for transfers, or 0 if the wallet is locked\n"
            "  \"paytxfee\": x.xxxx,                (numeric) the transaction fee configuration, set in " + CURRENCY_UNIT + "/kB\n"
            "  \"hdseedid\": \"<hash160>\"            (string, optional) the Hash160 of the HD seed (only present when HD is enabled)\n"
            "  \"hdmasterkeyid\": \"<hash160>\"       (string, optional) alias for hdseedid retained for backwards-compatibility. Will be removed in V0.18.\n"
            "  \"private_keys_enabled\": true|false (boolean) false if privatekeys are disabled for this wallet (enforced watch-only wallet)\n"
            "}\n"