Return more useful error when reaching /api/keystatic/github/created-app route when the GitHub App is already setup

[emmatown]

Fixes #1357

Note this only fixes the issue described in the title of #1357, the other problem described in the issue where there's a blank page in GitHub App creation flow seems to be something with Astro refreshing after Keystatic writes the env variables which I'm hoping to look into more

[changeset-bot]

🦋 Changeset detected

Latest commit: 3c37d55

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@keystatic/core	Patch
@keystar/docs	Patch
@example/astro-content	Patch
@example/astro	Patch
@example/localization	Patch
@example/next-app	Patch
@example/next-block-builder	Patch
@example/next-pages	Patch
@keystatic/remix-test-app	Patch
keystatic-docs	Patch
@keystatic/templates-astro	Patch
@keystatic/templates-nextjs	Patch
@keystatic/templates-remix	Patch

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