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

* Consider the darker/clang-format command to have failed if the exit
   code is non-zero, regardless of the stdout/stderr output.
* Propagate stderr from the formatter command to the script's
   caller (and into the GitHub log).
* On success, dump stdout to the caller, so it ends up in GitHub's log.
   I'm not sure what this would ever be, but if it exists, it should be
   preserved.
* Just before the script exits, if any formatter failed, print a line
   showing which formatters failed.