fix: don't retry failed implicit data preloads

[dummdidumm]

fixes #8301

Not sure if this is the best approach, therefore no test yet

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[Rich-Harris]

If we were to do this I think we'd want a failed_preloads.clear() somewhere, most likely after any successful navigation. I do somewhat lean towards arguing that this is the developer's responsibility to address with data-sveltekit-preload-data="off", though I could probably be persuaded in the other direction.

[dummdidumm]

I didn't add a clear because if you have a layout with a link, that link would try to be prefetched again (and fail again) after each navigation. Under what circumstances could the approach of only deleting the id in question lead to something not getting prefetched again? (possible answer: with a redirect; that probably needs a check to delete this as well)

If we don't want to change this, then we should at least print a more helpful message to the console at dev time. Something like "if this is a link that is very likely to fail while prefetching, consider setting data-sveltekit-preload-data="off" on it".

[Rich-Harris]

Under what circumstances could the approach of only deleting the id in question lead to something not getting prefetched again?

Say I fail to navigate to /admin, then navigate to /login, then attempt to navigate to /admin again. Following the successful navigation to /login I'd expect subsequent links to be preloaded

[dummdidumm]

Yeah that makes sense.

Since this is all highly app-specific, and hard to find a one-size-fits-all-solution (your case for example is more conservative, but the related issue would probably prefer my solution), I think a dev time warning is the better solution.