$message stays undefined on subsequent submission errors

[f-elix]

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Description
When using the message helper to return an error with a status code of 400 and above, the $message store is set on the first submission. On subsequent submissions, the store is cleared but it is never set to the returned message value again.

Repro here: https://www.sveltelab.dev/uvboaqr0th4q102. You can try submitting the form multiple times, and the message never reappears. Note that a 200 works fine.

[ciscoheat]

Probably a duplicate of #536, read there for a workaround, or reopen if not related.