Add error handling

...n/java/org/fuseri/model/dto/exercise/AnswerCreateDto.java

.../fuseri/model/dto/exercise/AnswerInQuestionCreateDto.java

.../java/org/fuseri/model/dto/exercise/AnswersCreateDto.java

...java/org/fuseri/model/dto/exercise/ExerciseCreateDto.java

...java/org/fuseri/model/dto/exercise/QuestionCreateDto.java

...java/org/fuseri/model/dto/exercise/QuestionUpdateDto.java

...va/org/fuseri/moduleexercise/answer/AnswerController.java

...n/java/org/fuseri/moduleexercise/exceptions/ApiError.java

...ava/org/fuseri/moduleexercise/exceptions/ApiSubError.java

.../fuseri/moduleexercise/exceptions/ApiValidationError.java

...ercise/exceptions/RestResponseEntityExceptionHandler.java

...rg/fuseri/moduleexercise/exercise/ExerciseController.java

...rg/fuseri/moduleexercise/question/QuestionController.java

...rg/fuseri/moduleexercise/exercise/ExerciseMapperTest.java