E502 - Group references unknown slide

Cause

A group's slides: list names a slide id that isn't declared in the deck's main slides: list.

Example

{ slides: [
  { id: "a", position: [0, 0], source: "a.slide.json" },
  { group: "demos", slides: ["a", "b"] }   // 'b' is not declared
]}

How to fix

Add the missing slide, or remove the unknown id from the group's slides: list.