[v23.1.x] rpk: improved err msg in rpk tune --output-script

[vbotbuildovich]

Backport #13473 to branch v23.1.x. Note that this issue was created as a placeholder, since the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v23.1.x
git checkout -b vbotbuildovich/backport-13473-v23.1.x-472
git cherry-pick -x 35e6b394bd53141e3257ee06535e1605a8a65b45

git push origin vbotbuildovich/backport-13473-v23.1.x-472
gh pr create \
  --title "[v23.1.x] rpk: improved err msg in rpk tune --output-script" \
  --base "v23.1.x" \
  --label "kind/backport" \
  --head "vbotbuildovich/backport-13473-v23.1.x-472" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --reviewer "" \
  --milestone "v23.1.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/13473 "

[r-vasquez]

Closed by #13546