Change default behaviour: client should abort if there is no backend

[copernico]

Currently the client proceeds silently if the backend is not reachable, however it is easy for the user to overlook the corresponding message.

To avoid confusion, the current default behaviour should be changed so that:

• by default client should abort if the backend does not respond with a 200
• the user can still change this via suitable switch --use-backend whose possible values are: always(default), optional (the current behaviour), no (do not even try to use the backend).