[STF] do not include CoreException in RMI calls as response

Because they cannot be serialized !

Change-Id: Id2acd07d17760409b1b52b6f0e4eb724fd3164e1
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1953
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>