Make “powerline-config” without arguments show proper message (python-3*)

In Python-3* it used to show the following:

    Traceback (most recent call last):
    File "/usr/lib/python-exec/python3.3/powerline-config", line 66, in <module>
        args.function(pl, args)
    AttributeError: 'Namespace' object has no attribute 'function'

now it shows

    usage: powerline-config [-h] {tmux,shell} ...
    powerline-config: error: too few arguments

in both python-2.7 and -3.3.

Fixes #1120.