<!-- 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.
pwallet->BlockUntilSyncedToCurrentChain();
    LOCK2(cs_main, pwallet->cs_wallet);
    
    CTxDestination dest = DecodeDestination(request.params[0].get_str());
    if (!IsValidDestination(dest)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
    }
    CScript scriptPubKey = GetScriptForDestination(dest);
    if (!IsMine(*pwallet, scriptPubKey)) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Address not found in wallet");
    }
    
    int nMinDepth = 1;
    if (!request.params[1].isNull())
        nMinDepth = request.params[1].get_int();
    
    CAmount nAmount = 0;
    for (const std::pair<const uint256, CWalletTx>& pairWtx : pwallet->mapWallet) {
        const CWalletTx& wtx = pairWtx.second;
        if (wtx.IsCoinBase() || !CheckFinalTx(*wtx.tx))
            continue;
        for (const CTxOut& txout : wtx.tx->vout)
            if (txout.scriptPubKey == scriptPubKey)
                if (wtx.GetDepthInMainChain() >= nMinDepth)
                    nAmount += txout.nValue;
    }
    return  ValueFromAmount(nAmount);
}
static UniValue getreceivedbylabel(const JSONRPCRequest& request)