Graceful error messages for the OK client 

See #184 for details. 
The basic concept is to display graceful error messages for when we know ok is going to crash. 

```
$ python ok
ok requires Python 3.
For more info: http://www-inst.eecs.berkeley.edu/~cs61a/fa14/lab/lab01/#installing-python
``
Just a slight nit. I think we're using lowercase `ok` :).