That's broken: it's supposed to report "warnings" (in yellow) rather
than "success" (in green) in that case.
The bot's config, , has a ShellCommandWithStderrWarning class that's
supposed to implement that behaviour, but sometime in the past few years
it stopped working. (I _think_ that was at a buildbot master upgrade,
but don't catch me on this.)
Could anyone look into this? We basically need to figure out how to
tell whether stderr is or isn't empty.