Fix exception: IOError instead of FileNotFoundError

The game checked for a nonstandard exception, which caused it to crash on my mac after I died:

```
You find yourself in a room.  Outside, you hear screams that sound something like a raptor ripping the flesh off it's live, struggling human prey.
You see 3 doors around you,and a desk in the corner of the room

0) Go to the first room.
1) Go to the second room.
2) Go to the third room.
3) Open the desk
1
You enter the room.

There is a raptor inside!
The raptor tears the flesh off your body and eats you.
You die.
The raptor dances over your grave.

Traceback (most recent call last):
  File "src/raptors.py", line 265, in <module>
    game = RaptorGame(game_dir)
  File "src/raptors.py", line 108, in __init__
    self.setup()
  File "src/raptors.py", line 192, in setup
    self.play_game()
  File "src/raptors.py", line 254, in play_game
    except FileNotFoundError:
NameError: global name 'FileNotFoundError' is not defined
```

I have substituted the correct exception; see [http://docs.python.org/2.7/library/exceptions.html].

It appears I've also shaven off some trailing whitespace as a side effect of my editor. My apologies for that distraction.