[llvm] [lit] Clean up internal shell parse errors with ScriptFatal (PR #68496)

[Vlad Serebrennikov via llvm-commits]

https://github.com/Endilll approved this pull request.

Even though I'd prefer to see more type annotations (e.g. on function parameters), I'm leaning towards them being out of scope of this PR. There is nothing wrong with type annotations that are added.

https://github.com/llvm/llvm-project/pull/68496