Fix misleading "Method not found" multiwallet errors

Raise RPC_WALLET_NOT_SPECIFIED instead of RPC_METHOD_NOT_FOUND when a required
wallet filename was not specified in an RPC call.

Also raise more specific RPC_WALLET_NOT_FOUND error instead of
RPC_INVALID_PARAMETER in case an invalid wallet was specified, for consistency.

src/bitcoin-cli.cpp

@@ -337,6 +337,10 @@ int CommandLineRPC(int argc, char *argv[])
 
                         if (errMsg.isStr())
                             strPrint += "error message:\n"+errMsg.get_str();
+
+                        if (errCode.isNum() && errCode.get_int() == RPC_WALLET_NOT_SPECIFIED) {
+                            strPrint += "\nTry adding \"-rpcwallet=<filename>\" option to bitcoin-cli command line.";
+                        }
                     }
                 } else {
                     // Result