Make Exception templates overrideable

[franklinkim]

Currently the exception template is only expected to be in the rendered app and not any parent classes:

private static function getExceptionTemplate($appClassName)
{
  // exception templates need to find parent stuff too ...
  $appExceptionTempateFile = self::getTemplateBase($appClassName) . DIRECTORY_SEPARATOR . 'exception.tpl';
  if (!file_exists($appExceptionTempateFile)) {
    $appExceptionTempateFile = \Foomo\ROOT . DIRECTORY_SEPARATOR . 'views' . DIRECTORY_SEPARATOR .  'Foomo' . DIRECTORY_SEPARATOR .  'MVC' . DIRECTORY_SEPARATOR . 'exception.tpl';
  }
  return new Template('Exception-' . $appClassName, $appExceptionTempateFile);
}

In the extended form you could also check for specific exception templates and then fall back to exception.tpl