The resources/templates/component/rss/bulleted_list/en.html template contains a couple of error messages to be used when the feed can't be displayed:
<p
petal:condition="true:self/is_5xx; true:self/user; true:self/user/is_editor"
>
<strong>
There is an RSS component which should go there, but it will not be displayed
because the RSS feed hasn't been fetched yet.
</strong>
</p>
<p
petal:condition="true:self/is_4xx; true:self/user; true:self/user/is_editor"
>
<strong>
There is an RSS component which should go there, but it will not be displayed
because the RSS feed which was specified was not found.
</strong>
</p>
But these never appear…