Ensure that DDM processing doesn't leave unhandled exceptions

Previously if a DDM chunk handler returns an invalid chunk the thread
that processed the chunk could end up with an unhandled exception.
This could be troublesome if it happened on a debugger thread, which
might never actually handle it.

Test: ./test.py --host -j50
Bug: 70570850
Change-Id: Ic67db951d721cf7532b08c0cc3f35bed9396d52d