docs(contributing): name the cause and the remediation in user-facing error messages

[kokhlo]

What does this PR do?

Adds the one-paragraph error-message convention to CONTRIBUTING.md as a bullet under Code Style, directly after the existing Error handling bullet (line 343 on current main): every user-facing error message names the actual cause and the remediation step, never the proximate symptom.

The text is lifted verbatim from #112005, which carries it as the deliverable following @teknium1's close comment on #111128 ("welcome as its own small docs PR"). Docs-only: one bullet, no code, no sweep — the individual message fixes stay on the per-issue path (#105150, #99831, #70908 and friends cite this convention once it lands).

Relationship to #111630

#111630 (open since earlier today) adds its own version of this convention as a standalone section further down the file, inside a PR that also carries four code fixes and tests. Per the one-cluster-per-PR rule quoted in the #111128 close note, this PR stays strictly docs-only and keeps the bullet form/placement the issue specifies. If the standalone-section form is preferred, I'm happy to rebase onto it or drop this — maintainer's call.

Closes #112005.

[alt-glitch]

This was generated by AI during triage.

Related: #111630 carries the same convention as a standalone section inside a larger code PR; this PR is the docs-only bullet form requested in the #111128 close note. Maintainer's call which form lands.

[teknium1]

Thanks @kokhlo — this landed on main via #112235 (4179c5a), which cherry-picked/salvaged this PR's work with your authorship preserved (your commit is on main as-is).

Closing in favour of the landed change. If anything from this PR is still missing on main, say so here and we'll pick it up.