<!-- 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("getwalletinfo", "")
            + HelpExampleRpc("getwalletinfo", "")
        );
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK2(cs_main, pwallet->cs_wallet);
    UniValue obj(UniValue::VOBJ);
    size_t kpExternalSize = pwallet->KeypoolCountExternalKeys();
    obj.pushKV("walletname", pwallet->GetName());
    obj.pushKV("walletversion", pwallet->GetVersion());
    obj.pushKV("balance",       ValueFromAmount(pwallet->GetBalance()));
    obj.pushKV("unconfirmed_balance", ValueFromAmount(pwallet->GetUnconfirmedBalance()));
    obj.pushKV("immature_balance",    ValueFromAmount(pwallet->GetImmatureBalance()));
    obj.pushKV("txcount",       (int)pwallet->mapWallet.size());
    obj.pushKV("keypoololdest", pwallet->GetOldestKeyPoolTime());
    obj.pushKV("keypoolsize", (int64_t)kpExternalSize);
    CKeyID seed_id = pwallet->GetHDChain().seed_id;
    if (!seed_id.IsNull() && pwallet->CanSupportFeature(FEATURE_HD_SPLIT)) {
        obj.pushKV("keypoolsize_hd_internal",   (int64_t)(pwallet->GetKeyPoolSize() - kpExternalSize));
    }
    if (pwallet->IsCrypted()) {
        obj.pushKV("unlocked_until", pwallet->nRelockTime);
    }
    obj.pushKV("paytxfee", ValueFromAmount(pwallet->m_pay_tx_fee.GetFeePerK()));
    if (!seed_id.IsNull()) {
        obj.pushKV("hdseedid", seed_id.GetHex());
        obj.pushKV("hdmasterkeyid", seed_id.GetHex());
    }