Revert r51725 (fall back to 127.0.0.1 when IP cannot be determined). On further discu...

Revert r51725 (fall back to 127.0.0.1 when IP cannot be determined). On further discussion, it's best if we don't make up a fake IP. Tweak the logic here a bit (per Tim) to let XFF attempt to determine the actual IP. Throw an exception if we can't.