Verify POS initialization inflight

[poszu]

We could catch if initialized labels are invalid very early on when they are created and before writing to a file. This would slow down initialization (how much depends on the % of labels checked), but give a higher confidence that the generated POS is valid and allow catching problems early.

It's essential that the problem of invalid labels is communicated to users in a way that is understandable so that they can take action by themself. We should not just panic with the message "labels are invalid" because users will be confused.