[z-machine] Returning 2 from @save in z5

[David Kinder]

> Suppose we load an arbitrary game from disk. The PC is pointing at the
> instruction after the @save which saved the game.

The Quetzal save file format spec makes the problem in the above explicit:

  5.8.2 On versions 4 and above, the SAVE instruction stores a value
   depending on the success of the save. The saved PC points to the single
   byte describing where to store the result.

David