<!-- 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.
"\nExamples:\n"
            + HelpExampleCli("rescanblockchain", "100000 120000")
            + HelpExampleRpc("rescanblockchain", "100000, 120000")
            );
    }
    WalletRescanReserver reserver(pwallet);
    if (!reserver.reserve()) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Wallet is currently rescanning. Abort existing rescan or wait.");
    }
    CBlockIndex *pindexStart = nullptr;
    CBlockIndex *pindexStop = nullptr;
    CBlockIndex *pChainTip = nullptr;
    {
        LOCK(cs_main);
        pindexStart = chainActive.Genesis();
        pChainTip = chainActive.Tip();
        if (!request.params[0].isNull()) {
            pindexStart = chainActive[request.params[0].get_int()];
            if (!pindexStart) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid start_height");
            }
        }
        if (!request.params[1].isNull()) {
            pindexStop = chainActive[request.params[1].get_int()];
            if (!pindexStop) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid stop_height");
            }
            else if (pindexStop->nHeight < pindexStart->nHeight) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, "stop_height must be greater than start_height");
            }