Better handling of preprocess errors

[benmccann]

Describe the problem

I was getting the error message:

[vite-plugin-svelte] Cannot read property 'split' of undefined
file: src/routes/__layout.svelte
Cannot read property 'split' of undefined

This was impossible to track down.

This turned out to be a bug in svelte-preprocess that appears to have been recently fixed. It happens if you have svelte-preprocess 4.9.0 with the following code:

<script lang="ts" context="module">
</script>
<script>
</script>

Here's a commit where it was happening: svelte-society/sveltesociety.dev@78cdfa1. You can reproduce with npm run build

Describe the proposed solution

It would be really nice to get a stack trace. I'm not sure why that was being swallowed and I was just getting the error message

Alternatives considered

We could put a try / catch around the call to preprocess to at least make it clear that's where it's coming from. svelte core could also do some type of reporting when a preprocessor fails. Neither solution would be as helpful as a stack trace though

Importance

would make my life easier

[dominikg]

We missed this when we implemented #218

I'll look into converting preprocessor errors in the same way. Let's hope there is frame info available

[benmccann]

There wouldn't be a frame in this particular case because that's something the Svelte compiler has to explicitly add and this was an unexpected error from a bug in svelte-preprocess. Maybe if there was a syntax error or something though

[dominikg]

fixed in 1.0.0-next.36