Render clear errors for unsupported Notion blocks

[hieu-lee]

Fixes #87.

GitPay task: https://gitpay.me/#/task/534/return-errors-when-importing-unsupported-content-types

Summary

• render a visible unsupported-block warning instead of an empty placeholder for missing/unsupported Notion block types
• handle collection_view records without collection data as unsupported, while preserving existing collection rendering when data is present
• add showUnsupportedBlockErrors={false} for the previous empty-placeholder behavior and renderUnsupportedBlock for custom fallbacks
• export UnsupportedBlock / getUnsupportedBlockMessage and document the new fallback behavior
• keep existing customBlockComponents behavior intact

Verification

• npm test -- --runInBand
• npm run build
• npm run lint (passes with existing warnings in src/components/asset.tsx and src/components/page-header.tsx)
• git diff --check