Add a log message if a campaign doesn't have a scenario matching firs…

…t_scenario

Otherwise this situation gives the WML coder merely the message "Failed to load
the scenario", without any further hint about why. The tagname is printed in case
the scenario needs to be a [multiplayer] or a [test] instead of a [scenario].

src/saved_game.cpp

@@ -280,6 +280,7 @@ void saved_game::expand_scenario()
 			update_label();
 			set_defaults();
 		} else {
+			ERR_NG << "Couldn't find [" << classification().get_tagname() << "] with id=" << carryover_["next_scenario"] << std::endl;
 			this->starting_point_type_ = STARTING_POINT_INVALID;
 			this->starting_point_.clear();
 		}