Make CI fail on check warnings, not just errors

[capnrefsmmat]

Currently the CI is set to allow check warnings:

epidatr/.github/workflows/ci.yml

Lines 34 to 39 in f9166b8

	- uses: r-lib/actions/check-r-package@v2
	env:
	DELPHI_EPIDATA_KEY: ${{ secrets.SECRET_EPIPROCESS_GHACTIONS_DELPHI_EPIDATA_KEY }}
	with:
	args: 'c("--no-manual", "--as-cran")'
	error-on: '"error"'

However, CRAN generally does not accept packages with check warnings unless you have a very good reason, so we should change this to reject warnings as well -- as soon as we fix our existing warnings, such as in #147.