Add a new state: USED_ERR

[dealfonso]

There is an ON_ERR state, but CLUES has no means to inform the user whether the whether the node being used or not. The user may check if the resources are used or not, but when considering the node to be recovered (e.g. trying to power it off again) it is tedious to check once and again whether the node is used or it is idle. A new state USED_ERR (and rename ON_ERR to IDLE_ERR) should be considered.