feat: add custom exception

src/io/takima/agencymanagement/dao/TravelDao.java

src/io/takima/agencymanagement/dao/UserDao.java

...anagement/exception/AlreadyTravelSubscribedException.java

src/io/takima/agencymanagement/model/Travel.java

src/io/takima/agencymanagement/service/TravelService.java

src/io/takima/agencymanagement/service/UserService.java