Some panics to unreachable!, better error msging

src/map_builders/room_based_starting_position.rs

@@ -20,7 +20,7 @@ impl RoomBasedStartingPosition {
             let start_pos = rooms[0].center();
             build_data.starting_position = Some(Position { x: start_pos.x, y: start_pos.y });
         } else {
-            panic!("RoomBasedStartingPosition only works after rooms have been created");
+            unreachable!("RoomBasedStartingPosition tried to run without any rooms.");
         }
     }
 }