* 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.
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