[INTERNAL] removed SarosCancellationException from Whiteboard

Change-Id: I5a7180fa68a28d3ead0b5063cd633d2a611ff979
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1941
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>