The stacktrace should be added to the statusReason field

Recently, we experienced NullPointerException, because of access problems to the
tmpdircommon directory):

StatusReason: Harvesting of seed (harvestname='webdanica-trial-1507138237796' failed due to exception: java.lang.NullPointerException

But the StackTrace should be included, as without this, it is not very useful.