[fix] Migration fixes

[dummdidumm]

• handle error without message, fixes Migration tool bombs out on sveltesociety.dev #6088
• automigrate status 200 in load
• handle GET without body (which is wrong, but people could have that)

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🦋 Changeset detected

Latest commit: 799467e

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

This PR includes changesets to release 1 package

Name	Type
svelte-migrate	Patch

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