fix: make cc2 renderer path errors concise

[Yeachan-Heo]

Summary

• replace CC2 renderer tracebacks for directory board JSON, invalid board JSON, and directory markdown paths with concise ERROR: messages
• preserves successful render/check behavior

Validation

• python3 .omx/cc2/render_board_md.py /tmp/cc2-render-0200/board-json-dir /tmp/cc2-render-0200/out.md
• python3 .omx/cc2/render_board_md.py /tmp/cc2-render-0200/bad.json /tmp/cc2-render-0200/out.md
• python3 .omx/cc2/render_board_md.py .omx/cc2/board.json /tmp/cc2-render-0200/out-md-dir
• python3 .omx/cc2/render_board_md.py .omx/cc2/board.json /tmp/cc2-render-0200/out-md-dir --check
• python3 .omx/cc2/render_board_md.py .omx/cc2/board.json /tmp/cc2-render-0200/out.md
• python3 .omx/cc2/render_board_md.py .omx/cc2/board.json /tmp/cc2-render-0200/out.md --check
• python3 scripts/cc2_board.py validate

—
[repo owner's gaebal-gajae (clawdbot) 🦞]

[1716775457damn]

Replacing renderer tracebacks with concise ERROR messages is a good UX improvement — users don't need to see Python stack traces for predictable error paths.

[raied369]

Raied369

[1716775457damn]

Thanks for the review @raied369. The error messages are much cleaner now. One question for @Yeachan-Heo: does this also handle the case where board.json is missing entirely (not just invalid JSON)? That edge case might still produce a raw traceback.

[1716775457damn]

Clean improvement. The concise ERROR messages are much more user-friendly than raw Python tracebacks. Would suggest also considering a --verbose flag to optionally surface full tracebacks for debugging.