threading : make stop the caller

[Laurent Claessens]

> Popping task off the end of the list is more efficient:

> while task_list:
>     task_list.pop().start()

That's cool. In my case it's better to do
task_list.pop(0).start

in order to pop the first element.

> or if the list is static

No, my list is dynamic and is feeded by an other thread (which also has 
to be closed).

Finally, I solved the problem by using a global flag looked at each 
iteration of the loop. A break is done if the global flag reports an error.

That was suggested by Alain Ketterlin from the French speaking python's 
usenet and Chris Angelico here.

Thanks all
Laurent