Change assertion yells while adding unknown hosts as replica set members.

Mongod yells assertion "bad config - dups?", "assertionCode"=>13278, "errmsg"=>"db assertion failure" while trying to add host unreachable/unknown (e.g. because of not working dns or simply because of error while typing the name) replica set member.

This is quite misleading and it should yell some other, better understandable assertion, e.g. "host unknown".