<!-- 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.
{ "rawtransactions",    "fundrawtransaction",               &fundrawtransaction,            {"hexstring","options","iswitness"} },
    { "wallet",             "abandontransaction",               &abandontransaction,            {"txid"} },
    { "wallet",             "abortrescan",                      &abortrescan,                   {} },
    { "wallet",             "addmultisigaddress",               &addmultisigaddress,            {"nrequired","keys","label","address_type"} },
    { "wallet",             "backupwallet",                     &backupwallet,                  {"destination"} },
    { "wallet",             "bumpfee",                          &bumpfee,                       {"txid", "options"} },
    { "wallet",             "createwallet",                     &createwallet,                  {"wallet_name", "disable_private_keys"} },
    { "wallet",             "dumpprivkey",                      &dumpprivkey,                   {"address"}  },
    { "wallet",             "dumpwallet",                       &dumpwallet,                    {"filename"} },
    { "wallet",             "encryptwallet",                    &encryptwallet,                 {"passphrase"} },
    { "wallet",             "getaddressesbylabel",              &getaddressesbylabel,           {"label"} },
    { "wallet",             "getaddressinfo",                   &getaddressinfo,                {"address"} },
    { "wallet",             "getbalance",                       &getbalance,                    {"dummy","minconf","include_watchonly"} },
    { "wallet",             "getnewaddress",                    &getnewaddress,                 {"label","address_type"} },
    { "wallet",             "getrawchangeaddress",              &getrawchangeaddress,           {"address_type"} },
    { "wallet",             "getreceivedbyaddress",             &getreceivedbyaddress,          {"address","minconf"} },
    { "wallet",             "getreceivedbylabel",               &getreceivedbylabel,            {"label","minconf"} },
    { "wallet",             "gettransaction",                   &gettransaction,                {"txid","include_watchonly"} },
    { "wallet",             "getunconfirmedbalance",            &getunconfirmedbalance,         {} },
    { "wallet",             "getwalletinfo",                    &getwalletinfo,                 {} },
    { "wallet",             "importaddress",                    &importaddress,                 {"address","label","rescan","p2sh"} },
    { "wallet",             "importmulti",                      &importmulti,                   {"requests","options"} },
    { "wallet",             "importprivkey",                    &importprivkey,                 {"privkey","label","rescan"} },
    { "wallet",             "importprunedfunds",                &importprunedfunds,             {"rawtransaction","txoutproof"} },
    { "wallet",             "importpubkey",                     &importpubkey,                  {"pubkey","label","rescan"} },
    { "wallet",             "importwallet",                     &importwallet,                  {"filename"} },
    { "wallet",             "keypoolrefill",                    &keypoolrefill,                 {"newsize"} },
    { "wallet",             "listaddressgroupings",             &listaddressgroupings,          {} },
    { "wallet",             "listlabels",                       &listlabels,                    {"purpose"} },
    { "wallet",             "listlockunspent",                  &listlockunspent,               {} },