Show errors to user if invalid scene data is loaded

[markmanx]

It's not difficult for a user to trigger a validation error when loading a scene. At the moment, Isoflow fails silently (errors can only be seen in the console). Users will appreciate seeing an error message if Isoflow can't load their scene (probably best displayed in a modal). We use Zod for input validation, and ideally we would let the user know where the error is in their input (e.g. connector with id {xxx} references a node which doesn't exist), but at this point that would be a nice-to-have.

At minimum Isoflow should display an error message if the scene fails to load.

[markmanx]

To recreate a validation error, pass in the following initialData object (validation will fail because the nodes property is missing):

const initialData = {
  icons: [],
  connectors: [],
  rectangles: []
}