<!-- 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->GetScriptForMining(coinbase_script);
    
    if (!coinbase_script) {
        throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, "Error: Keypool ran out, please call keypoolrefill first");
    }
    
    if (coinbase_script->reserveScript.empty()) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "No coinbase script available");
    }
    return generateBlocks(coinbase_script, num_generate, max_tries, true);
}
UniValue rescanblockchain(const JSONRPCRequest& request)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() > 2) {
        throw std::runtime_error(
            "rescanblockchain (\"start_height\") (\"stop_height\")\n"
            "\nRescan the local blockchain for wallet related transactions.\n"
            "\nArguments:\n"
            "1. \"start_height\"    (numeric, optional) block height where the rescan should start\n"
            "2. \"stop_height\"     (numeric, optional) the last block height that should be scanned\n"
            "\nResult:\n"
            "{\n"
            "  \"start_height\"     (numeric) The block height where the rescan has started. If omitted, rescan started from the genesis block.\n"
            "  \"stop_height\"      (numeric) The height of the last rescanned block. If omitted, rescan stopped at the chain tip.\n"
            "}\n"