text: Title: clean up tests and better error message with R notebooks
  Body: we need to address a few things from this PR: https://github.com/ploomber/ploomber/pull/1101
  
  1. refactor unit tests: https://github.com/ploomber/ploomber/pull/1101#discussion_r1212466632
  2. better error message when trying to run an R notebook with ploomber-engine https://github.com/ploomber/ploomber/pull/1101#discussion_r1211999094
  
  