Fallback to the default renderer if the config key is missing.

lib/Cake/Error/ErrorHandler.php

@@ -113,7 +113,7 @@ class ErrorHandler {
 		if (!empty($config['log'])) {
 			CakeLog::write(LOG_ERR, self::_getMessage($exception));
 		}
-		$renderer = $config['renderer'];
+		$renderer = isset($config['renderer']) ? $config['renderer'] : 'ExceptionRenderer';
 		if ($renderer !== 'ExceptionRenderer') {
 			list($plugin, $renderer) = pluginSplit($renderer, true);
 			App::uses($renderer, $plugin . 'Error');