Newbie question: how to pass errormessage from comment_insert() to comments()

[eluser]See64[/eluser]
I am working with the "blog example", but I can't figure out how to send a error message from the comment_insert() to the comments() part. I want to check for empty fields and tell the user he has to fill in something.

So instead of the redirect to /blog/comments, I want to go jump to comments() and pass an error message, so that I can send it along to the comment_view.