Use {cli} for messages/errors

[peterdesmet]

I suggest we use the cli package (we use in our packages). It allows:

• Nicely formatted error, warnings and messages
• Error classes (which makes testing easier)
• Much easier to create messages (no need for glue)

In my experience, it doesn't jive well with {assertthat}, so that's a dependency I would remove. Here's how typical cli message works (example from the frictionless R package):

  # Check resources
  if (length(package$resources) == 0) {
    cli::cli_abort(
      c(
        "{.arg package} must have resources.",
        "i" = "Use {.fun add_resource} to add resources."
      ),
      class = "frictionless_error_package_without_resources"
    )
  }

This would print:

And you can test it like:

expect_error(
  write_package(p_empty, dir),
  class = "frictionless_error_package_without_resources"
)