Error conditions.

  I assume that if a player tries an illegal move the game's supposed to inform
  the player of what went wrong. Can we output an error message using
  System.out.println, or do we need to set up errors that are thrown, or not
  report anything at all?

The game should detect illegal conditions and respond to them; returning a
status code or throwing an exception is fine.  Remember that you're writing a
back-end; you should not assume that the code has direct access to an output
device.