Raise exception after falling out the loop

Currently, register*node() does not cope well if the service fails
to come up, failing silently in the nova-bm case if the node only
has one MAC, and tossing a UnboundLocalError in the ironic case. Set
a guard value and raise an exception to protect against that.

Change-Id: I0b6c9a9f150dd1e9402ed3163d234f143d86ce9e