Hi there,
It's actually a super useful thing to be able to do, and, easy - once you know how.
First (and this seems unintuitive, I realize.) add the action right after your email that you want to happen if it fails.
Then, in your new action, go into the action settings and select "Configure run after" - and select only "failed".
You'll see the little arrow to your new action will turn into a red dashed line, indicating an error action.
You can also still have actions happen on success.
Let us know how you do!
Cheers,
Rhia