errbacks catch exceptions attached to deferreds. There are no exceptions that will skip over errbacks. If your errbacks are not being triggered, you're not putting them in the correct location in the deferred callback chain.
My mistake was to set log.error as errback in some locations, and that lead to the quoted messages without proper origin. After some hours of analyzing and refactoring I got rid of those stuff. Sorry for bothering you. Best regards, Henning Hraban Ramm Südkurier Medienhaus / MediaPro Support/Admin/Development Dept.