With `--unstage`, provide better error message(s)

[onnozweers]

I forgot to specify the file name to unstage, and got this error:

[onno@mylaptop ~/git/SpiderScripts] % ada/ada --netrc --unstage --request-id ea406339-8cb4-4ad4-8093-e359e61cbf4c
ERROR: unknown option 'ea406339-8cb4-4ad4-8093-e359e61cbf4c'. Use 'ada/ada --help' for more information.

Would be nice to provide a better error message. Not sure what yet, because --unstage can be used in several ways. Perhaps something like "With --unstage, you need to specify a file, directory or a list of files that you want to unstage".