Fixed incorrect method call on Ardent User object in AdminUsersController.php

As per the Ardent doccumentation (https://github.com/laravelbook/ardent#errors), line 79 of app/controllers/admin/AdminUsersController.php should read:

```
$error = $user->errors()->all();
```

instead of:
```
$error = $user->getErrors()->all(); 
```