Add a log message if a campaign doesn't have a scenario matching first_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 tag needs to be [multiplayer], [test] or [tutorial] instead of [scenario].

(cherry picked from commit d67e08826d)

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();
 		}