<!-- 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.
"If 'label' is specified, assign address to that label.\n"
            "\nArguments:\n"
            "1. nrequired                      (numeric, required) The number of required signatures out of the n keys or addresses.\n"
            "2. \"keys\"                         (string, required) A json array of bitcoin addresses or hex-encoded public keys\n"
            "     [\n"
            "       \"address\"                  (string) bitcoin address or hex-encoded public key\n"
            "       ...,\n"
            "     ]\n"
            "3. \"label\"                        (string, optional) A label to assign the addresses to.\n"
            "4. \"address_type\"                 (string, optional) The address type to use. Options are \"legacy\", \"p2sh-segwit\", and \"bech32\". Default is set by -addresstype.\n"
            "\nResult:\n"
            "{\n"
            "  \"address\":\"multisigaddress\",    (string) The value of the new multisig address.\n"
            "  \"redeemScript\":\"script\"         (string) The string value of the hex-encoded redemption script.\n"
            "}\n"
            "\nExamples:\n"
            "\nAdd a multisig address from 2 addresses\n"
            + HelpExampleCli("addmultisigaddress", "2 \"[\\\"16sSauSf5pF2UkUwvKGq4qjNRzBZYqgEL5\\\",\\\"171sgjn4YtPu27adkKGrdDwzRTxnRkBfKV\\\"]\"") +
            "\nAs json rpc call\n"
            + HelpExampleRpc("addmultisigaddress", "2, \"[\\\"16sSauSf5pF2UkUwvKGq4qjNRzBZYqgEL5\\\",\\\"171sgjn4YtPu27adkKGrdDwzRTxnRkBfKV\\\"]\"")
        ;
        throw std::runtime_error(msg);
    }
    LOCK2(cs_main, pwallet->cs_wallet);
    std::string label;
    if (!request.params[2].isNull())
        label = LabelFromValue(request.params[2]);
    int required = request.params[0].get_int();
    
    const UniValue& keys_or_addrs = request.params[1].get_array();