Improve template error messages for better developer experience

[Yenxji]

Currently this is for the Developer user only. When a template modification references a non-existent template, XenForo displays:


The problem:
This error message, while technically accurate, doesn't provide actionable guidance for developers, especially those new to XenForo development. The message simply states the template is "unknown" without explaining the potential causes or solutions.


Suggested improvement:
The error message should be more descriptive and helpful:

Template errors
* Template public:akltest_navigation: [E_USER_WARNING] Template 'public:akltest_navigation' does not exist or cannot be found.

Possible causes:
- The template has not been created yet
- The template was deleted or disabled
- The template name is misspelled in your template modification
- The addon providing this template is disabled or uninstalled

Suggested actions:
- Verify the template exists in Appearance > Templates
- Check your template modification for typos
- Create the template if it doesn't exist, or remove the modification referencing it

Benefits:

1. Improved developer experience - Clear guidance reduces debugging time
2. Better onboarding - New developers understand what went wrong and how to fix it
3. Reduced support requests - Self-explanatory errors mean fewer forum posts asking for help
4. Professional presentation - More polished error handling reflects well on the platform

This would align with modern development frameworks that prioritize developer experience through clear and actionable error messages.