Improve usability of `khal list -d absent`

[mgumz]

If a user provides an absent calendar name to be filtered out with khal list -d, an error is raised and the call stack is thrown at the user (24 lines of internals with a final "KeyError: 'absent').

To improve the usability, this improvement catches this situation and properly tells the user what happend and how to remedy the situation, just like -a absent would do.