Review all error messages

[maelle]

• Create helper functions if needed.
• Switch to cli::cli_abort().
• Make sure all messages end with a period.

Following up on #702

[krlmlr]

We have bigger problems, let's shelve for now.

[maelle]

#1693 (comment)

[maelle]

https://simonpcouch.github.io/chores/reference/cli_helper.html

[maelle]

After that we can put stop() in a list of unwanted functions fed to lintr.