Add test for webpack loader error handling
Test that errors thrown in webpack loaders surface the source file path
and error message in the CLI output. Covers four cases:
- Loader throws an Error object
- Loader throws a plain string (NonErrorEmittedError)
- Loader has an unhandled rejected Promise
- Loader has a setTimeout that throws after completion
https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4
