Auto merge of #135359 - RalfJung:lang-start-unwind, r=joboet

use a single large catch_unwind in lang_start

I originally planned to use `abort_unwind` but reading the comment in `thread_cleanup` it seems we are deliberately going for slightly nicer error messages here, so this preserves that. It still seems nice to not repeat `catch_unwind` so often.

Author: bors