prevent double-fixing of stack traces

[Rich-Harris]

fixes #3638. The problem here was that errors were having their stack traces 'fixed' (which is not an idempotent operation) twice in some cases, resulting in invalid source mappings.

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[Rich-Harris]

Couple of related issues: #3243 and #3371. #3243 isn't yet fixed by this PR, I haven't checked #3371 yet.

[Rich-Harris]

The test that's failing is because errors that occur during ssrLoadModule are already 'fixed', and we try to fix them again. Not sure what the most elegant way to prevent that is, will pick up tomorrow

[Rich-Harris]

Working around the fact that ssrLoadModule fixes stack traces is proving to be very difficult. Decided to skip that test and open vitejs/vite#7046 instead.