refactor(eventsub): use variants for sum types (#5930)

lib/twitch-eventsub-ws/src/errors.cpp

@@ -25,6 +25,8 @@ std::string ApplicationErrorCategory::message(int ev) const
             return "Missing inner root"s;
         case Kind::NoMessageHandler:
             return "No message handler found"s;
+        case Kind::UnknownVariant:
+            return "Unknown variant value"s;
     }
 
     assert(false);