Laravel: validation message for same rules in one field

I am new to Laravel, and I try to validate a request. I have to following request class:

namespace App\Http\Requests;

class TestRequest extends FormRequest
{
    protected function rules() 
    { 
        return [
            'group_id' => 'required|exists:groups,id,deleted_at,NULL|exists:group_users,group_id,user_id,' . \Auth::user()->id
        ];
    }
}

My problem is:

• I have to check that the group exists and it's not deleted. This is the first "exists" rule.
• And I have to check that the currently logged in user is part of the group. The second "exists" rule.

My question is:

• When any of the 2 exists fails, how do I know whitch one failed?
• I want to return a different error message for these exists checks. How should I do it?
• Do I have to write a custom validation for this?

PS: I'm using Laravel 5.3

I would recommend writing a custom rule. Check the below link for where to add it in your code

https://laravel.com/docs/5.3/validation#custom-validation-rules

Validator::extend('group_check', function($attribute, $value, $parameters, $validator) {
    // Do custom exists check 1;
    $group = Group::where('id', $value)->where('deleted_at', 'null')->first();

    if (!$group) {
        return false;
    }

    // Do custom exists check 2;
});

Validator::replacer('group_check', function($message, $value, $parameters, $validator) {
    // Do custom exists check 1 but instead of returning false, return a custom message

    // Do custom exists check 2 return a custom message
});