svn-warnings bot doesn't catch warnings

[Daniel Shahaf <d....@daniel.shahaf.name>]

https://ci.apache.org/builders/svn-warnings currently reports "success"
on builds that have compiler warnings, e.g.,
https://ci.apache.org/builders/svn-warnings/builds/1999/steps/Make/logs/stdio

That's broken: it's supposed to report "warnings" (in yellow) rather
than "success" (in green) in that case.

The bot's config, [1], 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.

Cheers,

Daniel

[1] https://svn.apache.org/repos/infra/infrastructure/buildbot/aegis/buildmaster/master1/projects/subversion.conf