function load_error_page(config) {
	let { errorTemplate } = config.kit.files;

	// Don't do this inside resolving the config, because that would mean
	// adding/removing error.html isn't detected and would require a restart.
	if (!fs.existsSync(config.kit.files.errorTemplate)) {
		errorTemplate = url.fileURLToPath(new URL('./default-error.html', import.meta.url));
	}

	return fs.readFileSync(errorTemplate, 'utf-8');
}
Loads the error page (src/error.html by default) if it exists.
Falls back to a generic error page content.
@param {import('types').ValidatedConfig} config