Improve epi_slide_{sum,mean} unsupported-arg messages.

[brookslogan]

Checklist

Please:

• Make sure this PR is against "dev", not "main" (unless this is a release
  PR).
• [-] Request a review from one of the current main reviewers:
  brookslogan, nmdefries.
• [-] Makes sure to bump the version number in DESCRIPTION. Always increment
  the patch version number (the third number), unless you are making a
  release PR from dev to main, in which case increment the minor version
  number (the second number).
  • Skipping bumping as this is so near 0.10.0 release PRs.
• [-] Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  1.7.2, then write your changes under the 1.8 heading).
  • should already be covered by the the epi_slide_opt change description
• See DEVELOPMENT.md for more information on the development
  process.

Change explanations for reviewer

• Also fixes lack of document() from accidental direct push. Branch protection settings have been updated to try to prevent.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves Fix check & better epi_slide_mean(new_col_name =) error message #580