Have the CLI require a backend to run

[copernico]

Currently, when the CLI is run and no backend is found, the operation continues with just a warning.
The result is not only a slower execution (because the benefit of reusing previously processed commits is lost) but also considerably lower accuracy, since the --use-nvd flag is practically disabled too -- remember: the NVD data feeds are accessed by the client through the backend APIs.
Because the warning message about the unreachable backend can easily be overlooked, the current behaviour can be confusing.

Proposal:

• change the default behaviour so that, if the backend is not reachable, the execution is aborted with a fatal error message.
• provide an explicit --no-backend flag, so that the user can explicitly indicate that they wish to use the tool without a backend

Your thoughts?