Template Part: Only display a missing notice in debug mode

[Mamaduka]

Description

Resolves #37222.

The "Template part not found" message was displayed for logged-in users. PR update condition to only show message when in debug mode.

It matches the condition for infinity recussion message.

P.S. Template Part block will still display the missing state in Site Editor.

How has this been tested?

1. Set WP_DEBUG and WP_DEBUG_DISPLAY to true.
2. In block theme temporarily rename the template-parts directory.
3. Check that the site displays messages.
4. Disable debug mode.
5. The site shouldn't display messages.

Types of changes

Bugfix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[kjellr]

This works in my testing! Thanks, @Mamaduka!

[talldan]

Ah, so it was only displaying for logged in users, which isn't too bad. But I think only in debug mode makes even more sense. Thanks @Mamaduka!