Log the type of a non-task after it executes

I can't actually reproduce the ClassCastException described in
issue 483.  So log the argument type if its not what we wanted
and we'll revisit the problem later once we have collected more
information about the state when it occurs.

Bug: issue 483
Change-Id: I5289ebf88570ebb997b443eb188167d77b61f0df
Signed-off-by: Shawn O. Pearce <sop@google.com>