make error.message enumerable

[Rich-Harris]

fixes #9010

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[benmccann]

Do we need to manually include err.frame as well or will that one be sent automatically? Not all errors include it, but compile errors do, so you can test by making bad syntax in a svelte template

[Rich-Harris]

I'm having a weirdly hard time reproducing the original error in order to try that out. When I was looking into it the other day though, there's a whole prepareError function that's used internally in Vite for a similar purpose, and if we wanted to add frame etc then we may as well lobby for that to be exposed so we can use it as well. But I couldn't create a situation in which an error with a frame would be created, so I figured that making message enumerable was probably enough. The important thing is that it no longer crashes silently. We could always try and add the extra fields later if we're able to figure out a situation in which they're relevant