Improve the error message of cbbackup if a node to backup is not in the cluster pool list

https://gist.github.com/4071813

If you look at this log, the user is backing up from IP address XXX.XXX.XXX.16 but this node is not in the pool list. (https://gist.github.com/4089474)

We do a bunch of work and at the end print out :
error: no design source node; spec_parts: ('192.168.42.16', '8091', 'Administrator', 'Administrator', '')

We should directly print out the error message without wasted work of backing up the nodes in the pool list.