Init/Error: Improve error handling in error.php

[mjansenDatabay]

This commit improves the global error handling in error.php:

1. Remove building a global template from tpl.main.html since it only contains one placeholder (see 2b8eafc) and provides no added value
2. Ensure a proper HTTP 500 status code is sent for normal error handling
3. Ensure a proper HTTP 500 status code is sent if an unexpected error occurs while rendering the error page
4. Log errors to error_log with minimal user-facing output, including an incident ID, timestamp, and message

[Copilot]

Pull Request Overview

This PR improves error handling in the global error.php file by streamlining error page rendering and enhancing error reporting. The changes focus on removing unnecessary template complexity, ensuring proper HTTP status codes, and implementing comprehensive error logging.

• Simplifies template handling by removing the global template wrapper and using only the error-specific template
• Adds proper HTTP 500 status codes for both normal error display and fallback error handling
• Implements structured error logging with incident IDs, timestamps, and detailed stack traces

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