There was a todo left to only catch MessageDeliveryFailure exceptions
once they were being propagated properly; the bug related to that [1]
has been closed and the fix is merged [2]. So we can catch only the
correct error and eliminate the TODO.
TrivialFix
[1] https://bugs.launchpad.net/neutron/+bug/1705351
[2] https://review.openstack.org/486706
Change-Id: I1201ee150709b84e6f4b2fa4b108da9ea28983a3