Dispatch exception throwed by job threads until mole script invoker

[dumoulin]

For now, if a task fails because of an exception, the exception isn't propagated to the call stack that have created the thread that manages the failing task.

We should find a way (Progress interface, new listener) for retrieving these execeptions at a higher level.

Note: it isn't annoying in the console as the exception is still printed in the output.