A task is put in an unusable state if the execution it belongs to is signalled

When an execution is signalled, the task belonging to that execution becomes unusable. It is made 'not-signalling', has no exection (thus cannot be ended, which you would not want to) but it keeps showing up in the tasklist....

Proposed solution is to have an additional task state (like the 'timeout' that is introduced). If an execution is signalled, the task is 'cancelled' this is something completely different then it being completed or any other state....

The behaviour of which transition will be taken will not be different from what it is now.... A warning will be logged that a execution is signalled and it would be better to act on the task itself...

Subtasks will/should be treated the same.