stack rollback failed: Conflict: resources.th_server: Cannot 'detach_interface' instance cfb79b55-ac54-4ee6-af94-ef7899fba9d8 while it is in vm_state error

[huangtianhua]

1. create a stack with a nova server
2. update the stack to change image of the server, and --rollback=true
3. stack rollback failed, because the flavor is too small for the new image, server rebuild failed, we can see
   the server in nova is ERROR, then heat started to rollback, and to detach ports from the existing server,
   an error raised: Conflict: resources.th_server: Cannot 'detach_interface' instance cfb79b55-ac54-4ee6-af94-
    ef7899fba9d8 while it is in vm_state error

Or maybe we don't have to detach/attach interfaces for an error server, give a message is enough?