Added error message for staff and superuser in visitors's login

[Rits1272]

Pull request checklist

Checklist:

• [x] I have performed a self-review of my own code
• [x] I have deployed locally to see if issue is fixed
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] Assigned current issues or new issues to PR

[Rits1272]

solved #23

[krritik]

This has merge conflicts due to your last PR. Please resolve them