Fix catch clause in PostRequestIndexing::destruct()

In \Drupal\search_api\Utility\PostRequestIndexing::destruct(), we were extra-diligent (or were trying almost neurotically to avoid yellow squiggly lines in PhpStorm – but definitely one of the two) and added a try/catch to $storage = $this->entityTypeManager->getStorage('search_api_index');.
However, for some reason we only catch InvalidPluginDefinitionException, even though PluginNotFoundException is actually more likely (which the accompanying comment also states).

Since we can’t use a multi-catch yet, I’d change it to just catch \Exception and leave a @todo comment for changing it to a multi-catch later.