The message construction was copied from another project doing a similar thing, so I had not spent too many seconds thinking how to do it. =)

The error message could also be improved further. Maybe something like:
> Cheffile exists but Librarian-Chef could not be loaded.
> Please add the librarian gem to your Gemfile or install it manually with `gem install librarian`.
> Or you can skip Librarian and this warning by using `--no-librarian` option or by putting `knife[:librarian] = false` to knife.rb.

But now off to bed...