Ninja Forms Custom Validation

I have a problem creating custom validation for my Ninja Forms. I am using something like this:

add_filter('ninja_forms_submit_data', 'custom_ninja_forms_submit_data');
function custom_ninja_forms_submit_data($form_data)
{
$field_id = 2;
$field_value = $form_data['fields'][$field_id]['value'];

$exists = true; // Check your database if $field_value exists

if($exists)
{
    $form_data['errors']['fields'][$field_id] = 'Value already exists';
}

return $form_data;
}

the problem is – my validation isn’t related to fields. I check some of the entered data and in case of an error – I need to show just general error message without blocking submit button. This code above shows error message below the field 2 and user can’t submit the form again without editing this field.