[llvm] llvm-reduce: Make some error messages more consistent (PR #133563)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 29, 3:06 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/133563).


https://github.com/llvm/llvm-project/pull/133563