Default message when git rate limited by failed auth attempts

What does this MR do and why?

Default message when git rate limited by failed auth attempts

Displays a static error message when requests are rate-limited due to too many failed git authentication attempts.

Extracts error class into its own file, since it now has more than just a class name to it.

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Before	After

(these screenshots made by running the same failing git fetch using an invalid token, but switching from my branch to master in another tab)

How to set up and validate locally

1. Ensure you have Rack Attack enabled locally in gitlab.yml :

   rack_attack:
     git_basic_auth:
       enabled: true
       maxretry: 10
       findtime: 60
       bantime: 3600
       ip_whitelist: ["838.98.1.0"] # make sure the default 127.0.0.1 doesn't apply

2. Clone a private project from your local install using https:// method
3. Add a new origin using the https url, but specifying an invalid HTTP basic username & password: git remote add pw https://admin:wrongpassword@YOUR_HOST:3443/group-slug/project-slug.git
4. Repeatedly fetch until you get banned: for ((i = 0; i < 100; i++)); do git fetch pw; done
5. Try to fetch from the (valid) origin remote: git fetch origin
6. You should see the message remote: Too many failed authentication attempts from this IP

Related to #22787 (closed)