Some of the buildbot scripts were not properly reporting an error during the
'clean' phase. Correct this.

Found by: danielsh

* tools/buildbot/slaves/centos/svnclean.sh,
  tools/buildbot/slaves/i686-debian-sarge1/svnclean.sh,
  tools/buildbot/slaves/svn-x64-macosx-gnu-shared-daily-ra_serf/svnclean.sh,
  tools/buildbot/slaves/svn-x64-macosx-gnu-shared/svnclean.sh,
  tools/buildbot/slaves/ubuntu-x64/svnclean.sh
    Exit with an error status if the 'make extraclean' step fails.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1463168 13f79535-47bb-0310-9956-ffa450edef68