Uncaught exceptions from webworkers started by tests in browser runner not shown properly

Describe the bug

When using the browser runner, if a test starts a webworker, and that webworker throws an uncaught exception, instead of the exception being properly printed with other test output, it gets shown as stderr output of "null" with no easy way to track down the source of it.

Reproduction

https://stackblitz.com/edit/vitest-dev-vitest-oszejgfz?file=test%2Fbasic.test.ts

System Info

N/A

See stackblitz.

Used Package Manager

npm

Validations

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• The provided reproduction is a minimal reproducible example of the bug.

Start with the StackBlitz reproduction linked in the issue, specifically test/basic.test.ts, and run it with the browser runner. Trace how uncaught webworker exceptions are collected and reported; done means the exception appears with the other test output and useful source information instead of as stderr from "null".