Raise missing template errors from `render_site_layout` and `render_menu`

[mamhoff]

What is this pull request for?

This raises an missing partial error in the test and development environments if there's a missing partial.

Notable changes (remove if none)

No changes in production, but better visibility on important errors in dev and test environments.

Checklist

• [ ] I have followed Pull Request guidelines
• [ ] I have added a detailed description into each commit message
• [ ] I have added tests to cover this change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.86%. Comparing base (f6b6ad5) to head (48be942).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2861 +/- ## ======================================= Coverage 95.86% 95.86% ======================================= Files 229 229 Lines 6211 6216 +5 ======================================= + Hits 5954 5959 +5 Misses 257 257 ```

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