BPMN21 — Clarify the possible catchers of error and escalation events

I always took for granted, that error and escalation events can only be catched in the same process instance at the same or a higher level as the throw event. A recent discussion in the OCEB group revealed, that the specification doesn't really support that view, without some reading between the lines.

The semantics of these important events should not be open to debate. I suggest to add a clarification.