Clarification and motivation

In #163 we have fixed the progress bar rendering thread to never abort in the middle of printing disregard when the links verification has finished. Now the terminal is not supposed to periodically appear in some weird state after xrefcheck completes.

However in case of an async exception (it may be a UserInterrupt which we now handle), AFAIS we still let the progress bar to be rendered partially, and this seems to be as bad as having a not cleaned up resource.

Acceptance criteria

• Even in case of exceptions we never render the terminal partially.
• Double ^C should still abort the application immediately.