[FEATURE] Check that all game slugs are unique when loading the games

[seternate]

Feature Request

User Story

As a Developer,
I want to validate at the start of the application (when loading the games) that the games have unique slugs,
so that I can not load different game definitions with the same slug and run into the problem that the wrong game definition is served through the REST interface.

Acceptance Criteria

• Loading different games with the same slug returns an error
• The error should be fatal and kill the application
• Proper error logging in place
  • both file names are displayed
  • state that they both have the same slug