Exit gracefully when cli args parsing fails

[leana8959]

Might be a nitpick but I think it's nice to not panic when it comes to cli option parsing which can be user errors.

Previously:

thread 'main' panicked at src/cli.rs:258:19:
Argument to option 'dnd' missing
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Current:

User error: Argument to option 'dnd' missing

[Toqozz]

I wholeheartedly agree. I'm not sure what I was thinking when I wrote that code.