<!-- 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.
bool p2sh = true;
    if (!request.params[1].isNull()) {
        p2sh = request.params[1].get_bool();
    }
    Witnessifier w(pwallet);
    bool ret = boost::apply_visitor(w, dest);
    if (!ret) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Public key or redeemscript not known to wallet, or the key is uncompressed");
    }
    CScript witprogram = GetScriptForDestination(w.result);
    if (p2sh) {
        w.result = CScriptID(witprogram);
    }
    if (w.already_witness) {
        if (!(dest == w.result)) {
            throw JSONRPCError(RPC_WALLET_ERROR, "Cannot convert between witness address types");
        }
    } else {
        pwallet->AddCScript(witprogram); 
        pwallet->SetAddressBook(w.result, "", "receive");
    }
    return EncodeDestination(w.result);
}
struct tallyitem
{
    CAmount nAmount;
    int nConf;
    std::vector<uint256> txids;
    bool fIsWatchonly;
    tallyitem()