Use {cli} for error messages

[EmilHvitfeldt]

I'm adding this issue as a reference point for future PRs as we properly are going to split them up a little for ease of review.

I'm thinking that the codebase can roughly be split into the following categories:

• environment control in aaa_models.R
• Model specific errors in boost_tree.R, c5_rules.R etc etc
• Everything else