… ToHtml, and tags

/claim #916

This PR improves HSX error messages by providing clearer and more helpful diagnostics for:

• Undefined variables
• Missing ToHtml instances
• Missing closing HTML tags
• Invalid tag names

The implementation includes:

• Contextual output
• Specific suggestions
• Cleaner formatting

Test file: ErrorDemo.hs Screenshot of test setup attached.

Let me know if a video demo is required — happy to provide one later.