<!-- 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.
+ HelpExampleRpc("getnewaddress", "")
        );
    if (pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Error: Private keys are disabled for this wallet");
    }
    LOCK(pwallet->cs_wallet);
    
    std::string label;
    if (!request.params[0].isNull())
        label = LabelFromValue(request.params[0]);
    OutputType output_type = pwallet->m_default_address_type;
    if (!request.params[1].isNull()) {
        if (!ParseOutputType(request.params[1].get_str(), output_type)) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("Unknown address type '%s'", request.params[1].get_str()));
        }
    }
    if (!pwallet->IsLocked()) {
        pwallet->TopUpKeyPool();
    }
    
    CPubKey newKey;
    if (!pwallet->GetKeyFromPool(newKey)) {
        throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, "Error: Keypool ran out, please call keypoolrefill first");
    }
    pwallet->LearnRelatedScripts(newKey, output_type);
    CTxDestination dest = GetDestinationForKey(newKey, output_type);
    pwallet->SetAddressBook(dest, label, "receive");
    return EncodeDestination(dest);
}
static UniValue getrawchangeaddress(const JSONRPCRequest& request)