Validate feature names in system_workers.yml

https://redmine.teklia.com/issues/10879#note-3

It is possible to declare a system worker for a non-existent feature in system_workers.yml. This worker will be entirely ignored by arkindex update_system_workers and by the system checks, but we get tricked into thinking it exists. The thumbnail generator added in !2531 (merged) is not a feature, so it never got automatically added.