<!-- 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() > 3)
        throw std::runtime_error(
            "listreceivedbylabel ( minconf include_empty include_watchonly)\n"
            "\nList received transactions by label.\n"
            "\nArguments:\n"
            "1. minconf           (numeric, optional, default=1) The minimum number of confirmations before payments are included.\n"
            "2. include_empty     (bool, optional, default=false) Whether to include labels that haven't received any payments.\n"
            "3. include_watchonly (bool, optional, default=false) Whether to include watch-only addresses (see 'importaddress').\n"
            "\nResult:\n"
            "[\n"
            "  {\n"
            "    \"involvesWatchonly\" : true,   (bool) Only returned if imported addresses were involved in transaction\n"
            "    \"amount\" : x.xxx,             (numeric) The total amount received by addresses with this label\n"
            "    \"confirmations\" : n,          (numeric) The number of confirmations of the most recent transaction included\n"
            "    \"label\" : \"label\"           (string) The label of the receiving address. The default label is \"\".\n"
            "  }\n"