discoverd/health: Make sure we clear hb on error
discoverd/health: Make sure we clear hb on error

If this is not done, on the next iteration we return and silently fail
on the next retry.


Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>
flynn/flynn,philiplb/flynn,lmars/flynn,benbjohnson/flynn,benbjohnson/flynn,philiplb/flynn,flynn/flynn,philiplb/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,benbjohnson/flynn,benbjohnson/flynn,benbjohnson/flynn,benbjohnson/flynn,flynn/flynn,flynn/flynn,lmars/flynn,flynn/flynn,lmars/flynn