Better handle DNSVS Webapi errors

[robinro]

Currently exceptions in calls to DNSVS are only reported with minimal output and then dropped. Improve this, by

• output the API error
• if exception is fatal, stop execution

[robinro]

While doing so: reduce code duplication in the webapi section.

[martingabelmann]

Better use https://git.scc.kit.edu/scc-net/netvs/netdb-client-lib directly which will reduce our code base by a lot.
However: requires python3.7 and OpenSuse still runs 3.6 (!!) so waiting might be worth it.

[AlexanderKaschta]

A small hint from the NETVS team: The netdb-client-lib relies on the net_api_generator, but the netdb-client-lib itself should run on Python 3.6. The generated classes of the net_api_generator however depend on the dataclass feature of Python, which has been introduced in Python 3.7. There is a backport for the dataclass feature for Python 3.6, which can be installed thru PyPI and should just get it running. If there are further questions, we are happy to support.