I was wondering if it would be possible to make a condition around a slot, to check if it is configured or not, and then display different markup.
Code example:
<isif condition="${The slot is configured}">
<isslot id="error-page-text-widget" context="folder" description="Error page text widget" context-object="${pdict.Widgets.contextFolder}" />
<iselse>
Other stuff
</isif>
Yes it is possible. You can also display another slot in a specific else condition. One thing to keep in mind is that included template slots aren't picked up by business manager. So you'll need to place them in the including template as well. But you can suppress their output in the including template using an <isif condition="${false}"></isif>
wrapped around it.