Tidy exception forwarding, warning consistency, and document conventions

[Copilot]

18 raise statements inside except blocks were missing from chaining, silently discarding original tracebacks. Three warning calls omitted the category argument and used inconsistent messages for the same condition. No documented convention existed for either.

Exception forwarding (chaining)

Every except ImportError/ModuleNotFoundError: that re-raised was updated to capture the original and chain it:

# Before
except ImportError:
    raise ImportError("tltk is not installed...")

# After
except ImportError as e:
    raise ImportError("tltk is not installed. Install it with: pip install tltk") from e

raise e in tools/misspell.py → bare raise (preserves original traceback; unused as e removed).

raise ValueError(str(e)) in corpus/core.py → raise ValueError(str(e)) from e.

Files: cli/benchmark.py, corpus/core.py, parse/{esupar,attaparse}_engine.py, spell/{symspellpy,phunspell,tltk}.py, tag/{tltk,pos_tag,thai_nner}.py, tokenize/{deepcut,tltk}.py, tools/misspell.py, translate/en_th.py, transliterate/tltk.py, util/abbreviation.py

Warning consistency

Three warnings.warn calls in phayathaibert/core.py and wangchanberta/core.py (two locations) had no explicit category and used three different phrasings for the same condition. All normalized to:

warnings.warn("This model does not support POS tag output.", UserWarning, stacklevel=2)

ImportError message normalization

All ImportError messages updated to the canonical form established by the new convention:

<Package> is not installed. Install it with: pip install <package>

CONTRIBUTING.md — new section: "Error messages, warnings, and exception handling"

Added under Code Guidelines to codify the conventions above for contributors and AI agents:

• Exception type selection table
• Exception message format (sentences, pip-install templates, param-value template)
• Exception forwarding rules (from e vs. from None vs. bare raise), with correct/incorrect examples
• Warning conventions (always explicit category + stacklevel, recommended categories table, message style)

Your checklist for this pull request

• Passed code styles and structures
• Passed code linting checks and unit test

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud