When executing `gh pr re` (note the incomplete command name), Cobra
would just display the help text for `gh pr` on standard output, exit
with status 0, and not print any message that you have mistyped the
"re" subcommand. Each part of this behavior is wrong.
This workaround makes sure that the helpful error message is printed on
stderr:
$ gh pr re
unknown command "re" for "gh pr"
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However, the exit status is still 0, whereas it should be non-zero.
Since `HelpFunc` does not return an error argument, we cannot trigger an
error status from this workaround.
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