Composition: Fix various CORS issues.

[tmeasday]

Closes #21500 Closes #21501

What I did

It seems that fetch() can throw directly for various CORS related errors, rather than returning a response with { ok: false }.

As we make 3 requests (index.json, stories.json, metadata.json) and it is common for servers to fail CORS when 404-ing, there's a good chance of hitting one of these (for instance Chakra does it on metadata.json, Chromatic does it on index.json <- 🐛 ).

So I:

• wrote some tests demonstrating the behaviour
• fixed the handling (it was a bit wonky anyway)

How to test

I used these three refs:

  refs: {
    foo: {
      title: 'Chakra',
      url:'https://storybook.chakra-ui.com/'
    },
    bar: {
      title: 'CH',
      url: 'https://59c59bd0183bd100364e1d57-jjsanukdmo.chromatic.com/'
    },
    baz: {
      title: 'local',
      url: 'http://localhost:8080/'
    }
  }

For the second you need access to the private repo and for the third you need to be running npx http-server storybook-static --cors

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[yarindeoh]

The fix was released in 7.0.0-rc.5 ?

[tmeasday]

7.0.0-rc.6 it looks like @yarindeoh: