add a wrapper that always throws with a custom error message

use that wrapper for lokid.jsonrpc

llarp/config/config.cpp

@@ -431,11 +431,19 @@ namespace llarp
     conf.defineOption<bool>(
         "lokid", "enabled", false, DefaultWhitelistRouters, AssignmentAcceptor(whitelistRouters));
 
-    auto setRPC = [this](std::string arg) { lokidRPCAddr = lokimq::address(arg); };
-
-    conf.defineOption<std::string>("lokid", "jsonrpc", false, DefaultLokidRPCAddr, setRPC);
+    conf.defineOption<std::string>(
+        "lokid",
+        "jsonrpc",
+        false,
+        "",
+        InvalidOption<std::string>("the [lokid]::jsonrpc option is deprecated please use the "
+                                   "[lokid]::rpc "
+                                   "config option instead"));
 
-    conf.defineOption<std::string>("lokid", "rpc", false, DefaultLokidRPCAddr, setRPC);
+    conf.defineOption<std::string>(
+        "lokid", "rpc", false, DefaultLokidRPCAddr, [this](std::string arg) {
+          lokidRPCAddr = lokimq::address(arg);
+        });
   }
 
   void

llarp/config/definition.hpp

@@ -448,4 +448,14 @@ namespace llarp
     return [&](T arg) mutable { ref = std::move(arg); };
   }
 
+  /// A wrapper that throws an invalid argument exception if the argument is provided in config
+  /// the invalid_arugment exception is constructed using a string that holds the exception's
+  /// message
+  template <typename T>
+  std::function<void(T)>
+  InvalidOption(std::string msg)
+  {
+    return [msg](T) mutable { throw std::invalid_argument(msg); };
+  }
+
 }  // namespace llarp