E501 - Edge references unknown slide

Cause

An edge endpoint names a slide id that isn't declared in the deck's slides: list.

Example

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

How to fix

Add the missing slide to the deck, or correct the endpoint id to match an existing slide.