wrap_instance_event() shouldn't swallow return codes

The wrap_instance_event() function, when used as a decorator,
will currently swallow return codes.  This prevents it from
being used on a function that returns a value that the caller
actually cares about.

Change-Id: I72616b0afcd3ab8fae81742bf4b7754cd959ef3a
Closes-Bug: 1311793
Merge "wrap_instance_event() shouldn't swallow return codes"