I think this issue is as old as kerosene. 
But seriously, it is indeed a very misleading "error", especially when you are not a Lustre expert.
Ideally, this situation would be a NOOP (no error message, etc.) if the node that is in standby for the target knew that it was active somewhere else. I'm not sure that's an easy problem though. I wonder if imperative recovery can help a standby node know this. Or given that the target it's complaining about should be shared with another node, MMP should be in place and maybe that could help the server, but that would require it knows which block device the target is on, which I would think it only ever knows when the target is started.
But even just clarifying the message might help. Maybe the message should be more english friendly. Something like "that target is not running on this node. perhaps it's running on $node" where I would guess it knows which other $node (ignoring that it could be on any number of other nodes for the moment) it could be running on by knowing which node(s) are failover for the target.
Brian I was thinking something along the lines of "If you are running an HA pair check that the target is mounted on the other server." Which puts the understanding of this error message back on the user rather than doing any code changes. It would, however, clarify what may be going on.
Patch submitted here http://review.whamcloud.com/7438
Patch landed to master as 1699de20882a56d6fda16f23897de0c0b4e2f610