Reduce log level,
child processes still running is a normal situation
if we killed the main process with SIGKILL.
Only if we do not manage to kill all processes this is a problem.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13539 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c