Reconsider use of JavaScript to inject 500 error details

[westonruter]

A 500.php template currently needs to include the following to display any 500 error that was returned from the server:

<details id="error-details" hidden>
	<summary><?php esc_html_e( 'More details', 'twentyseventeen-westonson' ); ?></summary>
	<iframe srcdoc=""></iframe>
	<script>
	function renderErrorDetails( data ) {
		if ( data.bodyText.trim().length ) {
			const details = document.getElementById( 'error-details' );
			details.querySelector( 'iframe' ).srcdoc = data.bodyText;
			details.hidden = false;
		}
	}
	</script>
	<?php wp_print_service_worker_error_details_script( 'renderErrorDetails' ); ?>
</details>

However, this currently depends on BroadcastChannel to send the message from the service worker to the client after the page has loaded:

https://github.com/xwp/pwa-wp/blob/57b295c646bfae63f78c3d735fbc37762709fca0/wp-includes/js/service-worker-error-response-handling.js#L11-L31

It would seem more straightforward to instead inject the error messages directly into the details element when the 500 error is being constructed. This could be done done with streams or more simply it could be done just by reading the body of the 500 error response and injecting it straight away.

[alcurrie]

Related #87 cc. @miina