`use:enhance` compatibility on form whose action isn't a SvelteKit action

Describe the problem

An unhelpful error message presents itself when we have a form with use:enhance posting to an ordinary POST endpoint instead of a SvelteKit action. This occurs because the enhance action tries to parse the fetch response body as a JSON. If that succeeds, it looks for a data property and attempts to parse it as JSON too, which can throw an error. If there is no data property, submitting the form results in nothing happening, which can be confusing.

https://stackblitz.com/edit/sveltejs-kit-template-default-nmxvlc?file=src%2Froutes%2F%2Blayout.svelte

Describe the proposed solution

Maybe we can handle the outcome better or have a dev-only warning when this occurs?

Alternatives considered

Document that use:enhance should only be used with SvelteKit actions

Importance

nice to have

Additional Information

No response

Start with the linked StackBlitz reproduction and inspect how the form uses use:enhance when posting to an ordinary POST endpoint. Trace the response handling to determine how this case should be surfaced, and consider the work done when submission no longer fails silently or produces an unhelpful error.