system_task_wrapper doesn't properly handle MultiError #50

system_task_wrapper has some code whose intention is to let some exceptions through (like Cancelled) and wrap others in TrioInternalError. But right now this logic totally fails to handle MultiErrors.

It's not super urgent, since this probably only comes up if someone makes a system task that hosts children and lets their exceptions out. But still, it should be fixed...

Relevant: #49