IO/Stata: clarify error when input is not a Stata dataset; add GitHub…

[oosei25]

Summary
Improve the error message raised by pd.read_stata when the input is not a
valid Stata dataset (e.g., users paste a GitHub /blob or /tree URL).
The new message is clearer about supported Stata versions and, when the input
looks like a GitHub page, appends a short hint to use the Raw file URL.

Motivation
Students frequently paste GitHub page URLs into read_stata, which previously
surfaced a confusing “Version of given Stata file is …” error. This PR makes
that failure mode actionable.

Behavior change

• Before: generic version/format error from the parser.
• After: explicit “not a valid Stata dataset … pandas supports versions …”
  message; if the string contains github.com with /blob or /tree, add:
  “Use the Raw file URL (replace /blob/ with /raw/ or click ‘Raw’).”

Implementation
Wrap the StataReader(...) construction in try/except ValueError. If the
message matches the existing “version/format” text, re-raise with the clearer
message. Append the GitHub hint only when the input path is a GitHub page URL.
No I/O is performed in the new logic; this only changes the error text.

Tests

• test_non_stata_gives_clear_message
• test_github_blob_hint_is_appended
• test_github_tree_hint_is_appended

Tests monkeypatch pandas.io.stata.StataReader to raise the original parsing
ValueError so there is no network dependency. All pass locally.

User-visible change?
Yes (error text), but no API change. No performance impact.

Docs
Optional; happy to add a short note in Stata I/O docs showing
wrong (/blob//tree) vs right (/raw or raw.githubusercontent.com) if preferred.

Checklist

• Tests added and passing locally
• pre-commit run -a clean locally
• No API changes; only clearer error text
• (optional) Add small docs note if maintainers want it