Failure to harvest Geodatenkatalog

We currently fail to harvest the Geodatenkatalog source erring out with

Dec 13 00:22:22 metadaten harvester[122951]: 2022-12-13T00:22:22.307706Z ERROR harvester: Failed to harvest 1 out of 10 sources
Dec 13 00:20:11 metadaten harvester[122951]: 2022-12-13T00:20:11.328306Z ERROR harvester: Failed to harvest source geodatenkatalog: custom error: missing field `CharacterString`

Beside the deserialization error, this is not really visible in the metrics

which just does not include the "geodatenkatalog" entry.

We should fix the deserialization error (probably by making the relevant field optional) and figure out a way to easily find the failed harvest in the metrics, e.g. a 0/0/1 entry.