Treat unknown exit codes as error

Change-Id: Ia9b119ea41b4b0230077cfc156d16e1490f07560
Reviewed-on: http://review.couchbase.org/30670
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>