Make `powerline-config` without arguments show proper message

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.

scripts/powerline-config

@@ -63,4 +63,12 @@ if __name__ == '__main__':
 
 	pl = config.create_powerline_logger(args)
 
-	args.function(pl, args)
+	try:
+		function = args.function
+	except AttributeError:
+		# In Python-3* `powerline-config` (without arguments) raises 
+		# AttributeError. I have not found any standard way to display same 
+		# error message as in Python-2*.
+		parser.error('too few arguments')
+	else:
+		function(pl, args)