[Ml-yokadi] Introducing BadUsageException

[Aurélien Gâteau]

Hi,

I just did some work fixing some unit-test breakage I created a few 
commits ago :/. While doing so, I realized bad usage (ie, wrong command 
line arguments) was not consistently handled, so I introduced a new 
exception: BadUsageException, which can be raised when the command does 
not contain correct arguments. It is caught by YokadiCmd, which will 
display the exception message as well as the command usage.

Aurélien