add inputvalidaiton an errors hints

build.gradle

...lin/de/thk/gm/gdw/todolist/controllers/TasksController.kt

...de/thk/gm/gdw/todolist/controllers/TasksRestController.kt

...lin/de/thk/gm/gdw/todolist/controllers/UsersController.kt

...de/thk/gm/gdw/todolist/controllers/UsersRestController.kt

src/main/kotlin/de/thk/gm/gdw/todolist/dtos/TaskDto.kt

src/main/kotlin/de/thk/gm/gdw/todolist/dtos/UserDto.kt

src/main/kotlin/de/thk/gm/gdw/todolist/models/User.kt

src/main/resources/templates/tasks/showTask.ftlh

src/main/resources/templates/tasks/showTasks.ftlh

src/main/resources/templates/users/showUsers.ftlh