Separately record sources which failed completely and fix handling of optional elements in CSW harvester

This allows us to expose this via e.g. Prometheus counters which can eventually be used for alerting.

The resulting metrics page before the CSW fix itself

and after applying the second commit

with the underyling issue being a missing format name which maps nicely to Rust's Option even though it drops the reason (but I have not found reason other than "unknown")

Closes #148 (closed)