Fix Shell on Python 3

[smarr]

As reported by @naomiGrew, the shell on Python 3 had issues with type confusion for the reading/writing to the terminal.

It resulted in an endless loop of the following output:

TypeError: a bytes-like object is required, not 'str'
Caught exception: a bytes-like object is required, not 'str'
Traceback (most recent call last):
  File "/Users/smarr/Projects/SOM/PySOM/src/som/vm/shell.py", line 23, in start
    stmt = raw_input("---> ")
           ^^^^^^^^^^^^^^^^^^
  File "/Users/smarr/Projects/SOM/PySOM/src/rlib/osext.py", line 33, in raw_input
    os.write(1, msg)
TypeError: a bytes-like object is required, not 'str'

This PR fixes it, and also adds special handling for ParseErrors and Exit in the shell to avoid printing the traceback for them.

/cc @naomiGrew @OctaveLarose