Ensure player pairings are deleted with rounds

Fix bug in previous bugfix ;-) - was recreating the files for rounds
Be less lenient on invalid hat round files

Throws an exception and terminates the app. Should only happen if
someone is playing around with the file, so it makes sense to warn
them something's up. I don't try to be too helpful.

Also:
- Rename TeamSet to HatRound
- Allow empty lines in players.csv