Handle external tool failures with actionable errors

uid: ISS-0014 status: BACKLOG priority: medium owner_role: REWORK estimate: TBD dependencies: [] risk: medium milestone: backlog

Category

REWORK

Evidence

• src/filare/render/pdf.py directly calls weasyprint.HTML(...).render().write_pdf(...) without catching ImportError, missing file errors, or weasyprint runtime failures; users see stack traces with no hint to install system deps (Pango/fonts).
• Graphviz rendering relies on the graphviz Python package; when dot is missing or not on PATH, graphviz.backend.ExecutableNotFound bubbles up with a raw trace. There is no Filare-side message pointing to Graphviz installation requirements.

Suggested Next Steps

1. Wrap PDF generation in try/except to emit a concise message: PDF generation failed: WeasyPrint/GTK/Pango not available or input HTML missing (<path>). Install system deps or drop format 'P'.
2. Detect ExecutableNotFound from Graphviz and rethrow a user-facing RuntimeError that mentions dot/Graphviz needs to be installed and on PATH (with a link to docs/README install section).
3. Add a minimal regression test that mocks missing Graphviz/WeasyPrint to assert the improved error strings (behavior unchanged except clearer messaging).