ExceptionHandlerController.java

    @ResponseBody
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<StringBuilder> processValidationError(HttpServletRequest httpServletRequest, MethodArgumentNotValidException manve) throws MethodArgumentNotValidException {
        StringBuilder response =  new StringBuilder();

        for (ObjectError oe : manve.getBindingResult().getAllErrors() ) {
            response.append(oe.getDefaultMessage() + "\n");
        }

        return new ResponseEntity<>(response, HttpStatus.BAD_REQUEST);
    }