Show stack trace for fatal errors if xdebug is loaded.

Backport of https://github.com/cakephp/cakephp/pull/5045 for 2.x
```php
<br>
<?= extension_loaded('xdebug') ? xdebug_print_function_stack() : null ?>
```
.... right below ```<?php echo h($error->getLine()); ?>```

?