Fallback to regular halt/1 if halt/2 is undefined.

Which is the case in R14.

Change-Id: I205fa969d93ae1e313d05c23539214c95b9c103a
Reviewed-on: http://review.couchbase.org/39228
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>