Fail prerendering if page attempts to access session in load - fixes #3722

[tcc-sejohnson]

It doesn't make sense to prerender a page that tries to access session, as it can never be populated by the server. Similarly to how we throw if we try to prerender a page that has a mutative shadow endpoint, we now throw if a page tries to access session during prerendering. This exception can be skipped by setting continueOnError to true, but it will still warn users by logging the error to the console. Fixes #3722.

As is, this will fail on server-side requests while in dev as well (so that developers don't have to wait until they try to build for prod to get slapped in the face with it).

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[tcc-sejohnson]

Hmm. Strange. I linted/checked locally...

I'll run again and push if necessary.

[Rich-Harris]

excellent, thank you!