Before only the output generated by extension add/remove/... functions
were logged to file, but not the error messages of unopkg itself.

Change-Id: I4451aa51e2877ae48c453ee4791e36d970832567
Reviewed-on: https://gerrit.libreoffice.org/62369
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>