[V3 Core] Setup should warn if replacing existing instance.

[Aioxas]

Please be sure to read through other issues as well to make sure what you are suggesting/reporting has not already
been suggested/reported

Type:

• Suggestion
• Bug

Brief description of the problem

Running redbot-setup with an existing redbot instance doesn't trigger any warnings, it just replaces it.

Expected behavior

Warnings before anything is done and options to cancel or continuing.

Actual behavior

Nothing, it continues as it is and replaces the existing instance values with the new instance values (same name)

Steps to reproduce

1. Run redbot-setup
2. follow the instructions on creating a new instance.
3. Make a new instance
4. Check config.json or the flags for details on the instances...is there even one?
5. Repeat Steps 2-4 for another instance and check.

[tekulvw]

Fixed with #1469