make a custom error message and prevent duplicate mail id from inserting in db - codeigniter-3

Question

I have checked email unique in codeigniter. In else past I diplayed error. But the system error is not convenient. how to customise error...
i have tried giveng javascript alert instead of that error message... but after that alert the data are storing in database.. Actually the data should not store in database if the email is not unique..
public function add_applicants() {
// $field_name=$this->input->post('photo');
$data = $this->input->post();
$mobile = $this->db->get_where('applicants',array('mobile_number'=>$this->input->post('mobile_number')));
if($mobile->num_rows()>0)
{
throw new Exception("Mobile Number Already Registered");
}
$email = $this->db->get_where('applicants',array('email_id'=>$this->input->post('email_id')));
if($email->num_rows()>0)
{
throw new Exception("Email_id Already Registered");
}
$school_id= $this->input->post('school_code');
$name_of_exam= $this->input->post('name_of_exam');
$name= $this->input->post('name_candiadte');
switch($name_of_exam){
case '1':
$exam='NMMS';
break;
case '2':
$exam='NTS';
break;
}
$school_code = $this->Welcome_Model->getschoolData($school_id);
if (isset($_FILES['photo']) == 1) {
$config['upload_path'] = FCPATH . 'uploads/'. $school_code->name_of_the_school.'/'. $exam ;
$this->upload_path = $config['upload_path'];
if ($this->validate_upload_path() == TRUE) {
$config['upload_path'] = $this->upload_path;
// print_r($this->upload_path) ;
// die();
}
$config['allowed_types'] = 'jpg';
$config['max_size'] = 2000;
$config['remove_spaces'] = TRUE;
$config['overwrite'] = true;
$config['file_name'] = $name.'-'.date('Ymdhis') . '.jpg';
$file_name = $config['file_name'];
$this->load->library('upload', $config);
$this->upload->initialize($config);
if (!$this->upload->do_upload('photo')) {
$this->upload->display_errors();
}
}
$data['photo'] = $file_name;
$insetapplicants = $this->Welcome_Model->insertApplicants($data);
if ($insetapplicants == 'Success') {
echo "<script>
alert('New Applicants Added Successfully');
// window.location.href='dashboard';
</script>";
}
}