How to display error message if record exists - validation

Question

I am trying to validate that if the email exists in my database, the error message will come out instead of the White Error Page.
I tried using this link but it is not working for me. Below are my codes.
Codes for Model Class
public class Employee {
#Id
#GeneratedValue (strategy = GenerationType.IDENTITY)
private Long id;
#Column(name = "first_name")
private String firstName;
#Column(name = "last_name")
private String lastName;
#Column(name = "email", unique = true)
#Email
private String email;
#Column(name = "posit")
private String position;
#Column(name = "mobile")
private String phone_num;
public Employee() {}
public Employee(String firstName, String lastName, String email, String position, String phone_num) {
super();
this.firstName = firstName;
this.lastName = lastName;
this.email = email;
this.phone_num = phone_num;
this.position = position;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getPosition() {
return position;
}
public void setPosition(String position) {
this.position = position;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getPhone_num() {
return phone_num;
}
public void setPhone_num(String phone_num) {
this.phone_num = phone_num;
}
}
Codes for EmployeeServiceInterface
public interface EmployeeService {
Employee save(Employee employee);
}
Codes for EmployeeServiceImplementation
#Service
public class EmployeeServiceImpl implements EmployeeService{
#Autowired
private EmployeeRepository employeeRepository;
#Override
public Employee save(Employee employee) {
this.employeeRepository.save(employee);
return employee;
}
public boolean exist(String email){
return employeeRepository.existsByEmail(email);
}
Controller
#Controller
public class EmployeeRegistrationController {
#Autowired
private EmployeeService employeeService;
private EmployeeServiceImpl employeeImpl;
#PostMapping("/saveEmployee")
public String saveEmployee(#ModelAttribute("employee") Employee employee) {
// save employee to database
employeeService.save(employee);
if(employeeService.exist==true){
return "User already exist";
}
return "success";
}
}