[llvm] [Workflow] Expand code-format-helper.py error reporting (PR #69686)

[Ryan Prichard via llvm-commits]

rprichard wrote:

One of my test PRs was supposed to fail C++ and Python formatting, but instead it only failed on C++, and it reproduced the failure mode where `darker` simply ignores code formatting errors:

https://github.com/rprichard/llvm-project/actions/runs/6592874295/job/17914345865?pr=4

Apparently darker is succeeding with no stdout/stderr output?

The darker command fails for me when I run it locally:
```
darker --check --diff -r e9a6609b7d4aedffa9109bb26ce1bee31d92dbb6..88be1bb44e96c38f550da9385f89aa427d4360b5 llvm/utils/git/test1.py
```

I still think this CL is an improvement on the error reporting, though. (I assume the latest fixup commit is OK.)

I used these PRs to test this PR so far: https://github.com/rprichard/llvm-project/pull/1 https://github.com/rprichard/llvm-project/pull/2 https://github.com/rprichard/llvm-project/pull/3 https://github.com/rprichard/llvm-project/pull/4 https://github.com/rprichard/llvm-project/pull/5

https://github.com/llvm/llvm-project/pull/69686