prevent unhandled rejections when loading page modules

[Rich-Harris]

fixes #3978. Even though the failure to load modules following a redeploy (which causes some filenames to be changed) was being dealt with (by causing the page to reload), rejections weren't being handled immediately before, when we preload modules to avoid waterfall.

This is a tricky thing to add a test for, but I verified that with this change the unhandledrejection event no longer happens, which should clean up Sentry logs etc.

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🦋 Changeset detected

Latest commit: 83dddcf

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

This PR includes changesets to release 1 package

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@sveltejs/kit	Patch

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[ghostdevv]

Is there anything blocking this being merged? 🙏

[Rich-Harris]

nope, nothing blocking except me being in a place with bad wifi for the last couple of weeks — still playing catch-up!

[ghostdevv]

@Rich-Harris yikes! no worries tysm!