manager: catch exceptions in mainloop

This patch prevents exceptions to crash application
if happen during pollEvents() processing.
Exception are just catch and displayed.
In case of exception caused by a task, this one
is removed from pending list.
Handlers are not removed, take care of that!

Change-Id: I266539585baf68f329d53f3d20361c4cb08211f3
Tuleap: #200