telling users they will need to register before commenting

hi
just setting up and have decided to require users to register before able to post…. have placed the settings accordingly, yet two undesirables occur:

1. user can happily complete comment section and click ‘post’… only then does a error 500 page come up… I would like the register page to appear on click post (with added text above the fill-in block), with: something like ”please register to comment”

2. to inform the user the need to be registered and logged in before commenting, I would like a default note within the comment box to this effect.

thanks for any guidence on this one
cheers
stephenn