raise more suitable errors upon Hets.choose
raise more suitable errors upon Hets.choose

This contains two error classes, one that is to be raised if there is no
HetsInstance at all registered with the application and the other is to
be raised if there is no *useable* HetsInstance to be find (that means
an instance that is both 'up' (read: reachable) and has an acceptable
minimal hets revision.

ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub,ontohub/ontohub