[INTERNAL] ConnectionStateComposite did not display the correct errors

Due to changes made in the SarosNet class the returned error was always
null and so it was not possible to show correct error messages.

Change-Id: I7324016d78186416b2dcf3f524e002dd88abb750
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1131
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>